Core: add variant builder implementation

[aihuaxu]

This PR adds the implementation to build a variant for primitive, object and array.

The usage:

• Build from Json
  VariantBuilder.parseJson(json_string);

• build a primitive

VariantBuilder builder = new VariantBuilder();
VariantPrimitiveBuilder primBuilder = builder.createPrimitive();
Variant variant = primBuilder.writeNumeric(10).build();

• build an object

 VariantBuilder builder = new VariantBuilder();
 VariantObjectBuilder objBuilder = builder.startObject();
 objBuilder.writeString("key1", "value1").writeNumeric("key2", 3).endObject();
 Variant variant = builder.build();

• build an array

 VariantBuilder builder = new VariantBuilder();
 VariantArrayBuilder arrBuilder = builder.createArray();
 arrBuilder.writerNumeric(10).writerNumeric(20).endArray();
 Variant variant = builder.build();

Nested arrays and objects can be supported as well. Please see the test cases for the usage. Compared to build from JSON string, we also support some additional types such as timestamp, date.

Part of: #10392

[aihuaxu]

@rdblue, @RussellSpitzer Please help review. Thanks.

[rdblue]

There are existing utilities in JSONUtil for a lot of the boilerplate around parsing and generating JSON. Is there a reason not to use the same approach here?

[aihuaxu]

We need to parse the JSON token by token so we can encode them in binary. Seems we only have the version to parse the JSON string into a full JSONNode and then create the object.

[xxubai]

Suggested change

	valueBuffer.writePrimitive(Variants.PhysicalType.NULL, null);
	valueBuffer.writePrimitive(PhysicalType.NULL, null);

[github-actions]

This pull request has been marked as stale due to 30 days of inactivity. It will be closed in 1 week if no further activity occurs. If you think that’s incorrect or this pull request requires a review, please simply write any comment. If closed, you can revive the PR at any time and @mention a reviewer or discuss it on the dev@iceberg.apache.org list. Thank you for your contributions.

[github-actions]

This pull request has been closed due to lack of activity. This is not a judgement on the merit of the PR in any way. It is just a way of keeping the PR queue manageable. If you think that is incorrect, or the pull request requires review, you can revive the PR at any time.