Accept missing "configuration" key for "chunkKeyEncoding".

The DefaultChunkKeyEncoding constructor currently requires a
`Configuration` argument, which means that the Jackson deserializer will
fail upon attempting to parse array metadata containing a
`chunk_key_encoding` like this one:

```
"chunk_key_encoding": {
  "name": "default"
}
```

Instead, it expects, at a minimum:

```
"chunk_key_encoding": {
  "name": "default",
  "configuration": {}
}
```

I believe Zarr-Java is being too strict here. It is true that that the
spec for the "Default chunk key encoding" [1] says nothing about the
`configuration` object being optional (it only says that the `separator`
member is optional). But the spec for extension points [2] (and
`chunk_key_encoding` _is_ an extension point) does say that the
`configuration` member MAY be present (and therefore is optional), and
in the absence of any overriding statement, I believe `configuration`
should be considered optional _also_ in the context of the "Default
chunk key encoding" extension.

There are definitely OME-Zarr files out in the wild with a missing
`configuration` member, for example [3].

This commit relaxes the requirement in the constructor of both the
DefaultChunkKeyEncoding and V2ChunkKeyEncoding classes, so that they
accept a missing (null) configuration argument and default to using a
'/' and '.' separator, respectively.

--
[1] https://zarr-specs.readthedocs.io/en/latest/v3/chunk-key-encodings/default/index.html
[2] https://zarr-specs.readthedocs.io/en/latest/v3/core/index.html#extension-definition
[3] https://uk1s3.embassy.ebi.ac.uk/idr/zarr/v0.5/idr0066/ExpD_chicken_embryo_MIP.ome.zarr

Author: gouttegd

src/main/java/dev/zarr/zarrjava/v3/chunkkeyencoding/DefaultChunkKeyEncoding.java

@@ -19,9 +19,13 @@ public class DefaultChunkKeyEncoding extends ChunkKeyEncoding {
 
   @JsonCreator
   public DefaultChunkKeyEncoding(
-      @Nonnull @JsonProperty(value = "configuration", required = true) Configuration configuration
+      @JsonProperty(value = "configuration") Configuration configuration
   ) {
-    this.configuration = configuration;
+      if (configuration == null) {
+        this.configuration = new Configuration(Separator.SLASH);
+      } else {
+        this.configuration = configuration;
+      }
   }
 
 

src/main/java/dev/zarr/zarrjava/v3/chunkkeyencoding/V2ChunkKeyEncoding.java

@@ -19,9 +19,13 @@ public class V2ChunkKeyEncoding extends ChunkKeyEncoding {
 
   @JsonCreator
   public V2ChunkKeyEncoding(
-      @Nonnull @JsonProperty(value = "configuration", required = true) Configuration configuration
+      @JsonProperty(value = "configuration") Configuration configuration
   ) {
-    this.configuration = configuration;
+      if (configuration == null) {
+        this.configuration = new Configuration(Separator.DOT);
+      } else {
+        this.configuration = configuration;
+      }
   }
 
   @Override