Adds a new CoderTranslator for Java SchemaCoders.

[acrites]

The new translator splits out the Schema proto from the toRowFn and fromRowFn's so that runners can check the Schema proto for compatibility updates such as reordering fields. Previously, the coder was simply translated as a custom java coder, which runners couldn't reason about.

Since this is a potentially backwards incompatible change, this new coder format is guarded by the "use_known_schema_coder" PipelineOptions experiment.

This fixes a few issues related to Schemas and pipeline updates: addresses #36496, #30276, #29245.

This PR is essentially a roll forward of the rollback #38514 with the change protected by experiment instead of using the pipeline compatibility version number.

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[acrites]

@kennknowles and @scwhittle

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