[BUG] Add _size to PutMappingRequest

[atoms42]

What is the bug?

The _size field is not present on PutMappingRequest, despite being present on TypeMapping.

This prevents enabling size tracking for existing indices.

How can one reproduce the bug?

See https://github.com/opensearch-project/opensearch-java/blob/main/java-client/src/main/java/org/opensearch/client/opensearch/indices/PutMappingRequest.java

What is the expected behavior?

_size (or SizeField as the Java classes call it) is present on PutMappingRequest

Do you have any additional context?

• This field is available on the dotnet version of the client
• It would be nice if PutMappingRequest used the same TypeMapping object used for index creation.