Add 3DTILES_time_dynamic extension

extensions/3DTILES_time_dynamic/README.md

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+# 3DTILES_time_dynamic
+
+## Contributors
+
+* Sean Lilley, Cesium
+
+## Status
+
+Draft
+
+## Dependencies
+
+Written against the 3D Tiles 1.1 specification.
+
+## Optional vs. Required
+
+This extension is required, meaning it must be placed in both the `extensionsUsed` and `extensionsRequired` lists in the tileset JSON.
+
+## Overview
+
+This extension adds support for time-dynamic 3D Tiles through two new [content metadata semantics](https://github.com/CesiumGS/3d-tiles/blob/main/specification/Metadata/Semantics/README.adoc#content):
+
+Semantic | Type | Description
+--|--|--
+`CONTENT_TIMESTAMP_START`| Type: `STRING` | The start time (inclusive). Should be stored as Coordinated Universal Time (UTC) using ISO 8601 format.
+`CONTENT_TIMESTAMP_STOP`| Type: `STRING` | The stop time (exclusive). Should be stored as Coordinated Universal Time (UTC) using ISO 8601 format.
+
+Together `CONTENT_TIMESTAMP_START` and `CONTENT_TIMESTAMP_STOP` form a time interval during which the content should appear.
+
+This extension is often used with [multiple contents](https://github.com/CesiumGS/3d-tiles/blob/main/specification/README.adoc#core-tile-content) where each content represents a keyframe on the tile's timeline.
+
+## Explicit Tiling Example
+
+This example shows a root tile with three contents.
+
+The first content appears at `2024-11-01`, the second content appears at `2024-11-02`, and the third content appears at `2024-11-03`.
+
+_Tileset JSON_
+
+```jsonc
+{
+  "asset": {
+    "version": "1.1"
+  },
+  "schema": "schema.json",
+  "geometricError": 1000,
+  "root": {
+    "boundingVolume": {
+      "region": [-1.318, 0.697, -1.319, 0.698, 0, 20]
+    },
+    "refine": "ADD",
+    "geometricError": 0,
+    "contents": [{
+      "uri": "content/day_0.glb",
+      "metadata": {
+        "timestampStart": "2024-11-01T00:00:00Z",
+        "timestampStop": "2024-11-02T00:00:00Z"
+      }
+    }, {
+      "uri": "content/day_1.glb",
+      "metadata": {
+        "timestampStart": "2024-11-02T00:00:00Z",
+        "timestampStop": "2024-11-03T00:00:00Z"
+      }
+    }, {
+      "uri": "content/day_1.glb",
+      "metadata": {
+        "timestampStart": "2024-11-03T00:00:00Z",
+        "timestampStop": "2024-11-04T00:00:00Z"
+      }
+    }]
+  },
+  "extensionsUsed": ["3DTILES_time_dynamic"],
+  "extensionsRequired": ["3DTILES_time_dynamic"]
+}
+```
+
+_Schema JSON_
+
+```jsonc
+{
+  "id": "exampleSchema",
+  "classes": {
+    "content": {
+      "properties": {
+        "timestampStart": {
+          "type": "STRING",
+          "required": true,
+          "semantic": "CONTENT_TIMESTAMP_START"
+        },
+        "timestampStop": {
+          "type": "STRING",
+          "required": true,
+          "semantic": "CONTENT_TIMESTAMP_STOP"
+        }
+      }
+    }
+  }
+}
+```
+
+## Implicit Tiling Example
+
+A similar example using implicit tiling. Note that three extensions are being used here:
+
+* `3DTILES_time_dynamic`
+* `3DTILES_implicit_tiling_multiple_contents`
+* `3DTILES_implicit_tiling_custom_template_variables`
+
+_Tileset JSON_
+
+```jsonc
+{
+  "asset": {
+    "version": "1.1"
+  },
+  "schema": "schema.json",
+  "geometricError": 1000,
+  "root": {
+    "boundingVolume": {
+      "region": [-1.318, 0.697, -1.319, 0.698, 0, 20]
+    },
+    "refine": "ADD",
+    "geometricError": 5000,
+    "content": {
+      "uri": "content/{timestampStart}_{timestampStop}.glb"
+    }
+  },
+  "extensionsUsed": ["3DTILES_time_dynamic", "3DTILES_implicit_tiling_multiple_contents", "3DTILES_implicit_tiling_custom_template_variables"],
+  "extensionsRequired": ["3DTILES_time_dynamic", "3DTILES_implicit_tiling_multiple_contents", "3DTILES_implicit_tiling_custom_template_variables"]
+}
+```
+
+_Schema JSON_
+
+```jsonc
+{
+  "id": "exampleSchema",
+  "classes": {
+    "tile": {
+      "properties": {
+        "contentCount": {
+          "type": "SCALAR",
+          "componentType": "UINT16",
+          "semantic": "TILE_CONTENT_COUNT"
+        }
+      }
+    },
+    "content": {
+      "properties": {
+        "timestampStart": {
+          "type": "STRING",
+          "required": true,
+          "semantic": "CONTENT_TIMESTAMP_START"
+        },
+        "timestampStop": {
+          "type": "STRING",
+          "required": true,
+          "semantic": "CONTENT_TIMESTAMP_STOP"
+        }
+      }
+    }
+  }
+}
+```
+
+_Subtree JSON_
+
+```json
+{
+  "buffers": [...],
+  "bufferViews": [...],
+  "propertyTables": [
+    {
+      "class": "tile",
+      "count": 4,
+      "properties": {
+        "contentCount": {
+          "values": 2
+        }
+      }
+    },
+    {
+      "class": "content",
+      "count": 5,
+      "properties": {
+        "timestampStart": {
+          "values": 3,
+          "stringOffsets": 4,
+        },
+        "timestampStop": {
+          "values": 5,
+          "stringOffsets": 6
+        }
+      }
+    }
+  ],
+  "tileAvailability": {
+    "bitstream": 0,
+    "availableCount": 4
+  },
+  "contentAvailability": [{
+    "bitstream": 1,
+    "availableCount": 2
+  }],
+  "childSubtreeAvailability": {
+    "constant": 0
+  },
+  "tileMetadata": 0,
+  "contentMetadata": [1]
+}
+```