Add ml-model:type field for API filtering

Author: duckontheweb

README.md

@@ -55,7 +55,8 @@ these models for the following types of use-cases:
 
 | Field Name                 | Type                      | Description |
 | -------------------------- | ------------------------- | ----------- |
-| ml-model:learning_approach | string                    | **REQUIRED**. The learning approach used to train the model. It is STRONGLY RECOMMENDED that you use one of the values [described below](#ml-modellearning_approach), but other values are allowed. |
+| ml-model:type              | string                    | **REQUIRED.** This MUST always be the constant `"ml-model"`. This purpose of this field is to provide a convenient way to filter ML Model Items in a STAC API. |
+| ml-model:learning_approach | string                    | **REQUIRED.** The learning approach used to train the model. It is STRONGLY RECOMMENDED that you use one of the values [described below](#ml-modellearning_approach), but other values are allowed. |
 | ml-model:prediction_type   | string                    | **REQUIRED.** The type of prediction that the model makes. It is STRONGLY RECOMMENDED that you use one of the values [described below](#ml-modelprediction_type), but other values are allowed.   |
 | ml-model:architecture      | string                    | **REQUIRED.** Identifies the architecture employed by the model (e.g. RCNN, U-Net, etc.). This may be any string identifier, but publishers are encouraged to use well-known identifiers whenever possible. |
 | ml-model:training-environment | [Training Environment Object](#training-environment-object) | Describes the environment used to train the model. See the Link [relation types](#relation-types) defined below for definitions of the data used during training. |

examples/cyclone-wind-speed/item.json

@@ -5,7 +5,7 @@
     "https://stac-extensions.github.io/scientific/v1.0.0/schema.json"
   ],
   "type": "Feature",
-  "id": "model-item",
+  "id": "model-cyclone-wind-estimation-torchgeo",
   "bbox": [
     -179.999,
     -4.5,
@@ -78,6 +78,7 @@
         "url": "https://www.microsoft.com/en-us/ai/ai-for-good"
       }
     ],
+    "ml-model:type": "ml-model",
     "ml-model:learning_approach": "supervised",
     "ml-model:prediction_type": "regression",
     "ml-model:architecture": "resnet18",

examples/dummy/item.json

@@ -52,6 +52,7 @@
         "url": "https://nrcan-rncan.gc.ca"
       }
     ],
+    "ml-model:type": "ml-model",
     "ml-model:learning_approach": "supervised",
     "ml-model:prediction_type": "object-detection",
     "ml-model:architecture": "RCNN",

json-schema/schema.json

@@ -26,6 +26,7 @@
                 {
                   "$comment": "Require fields here for Item Properties.",
                   "required": [
+                    "ml-model:type",
                     "ml-model:learning_approach",
                     "ml-model:prediction_type",
                     "ml-model:architecture"
@@ -169,6 +170,9 @@
       "$comment": "Add your new Item fields here. Don't require them here, do that above in the corresponding schema.",
       "type": "object",
       "properties": {
+        "ml-model:type": {
+          "const": "ml-model"
+        },
         "ml-model:learning_approach": {
           "type": "string"
         },