fix failing ProtoSchemaTranslator on proto3 optional fields

[udayaw]

I found that ProtoSchemaTranslator fails to translate the schema when proto3 definitions have optional fields.

These optional fields creates synthetic oneOfs internally and ProtoSchemaTranslator runs into errors treating them are real oneOfs. This excludes that from happening.

I had to use getRealContainingOneof() since they have deprecated isSynthetic()

Fixes #32199

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[udayaw]

Run Java PreCommit

[udayaw]

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Reminder, please take a look at this pr: @Abacn

[Abacn]

testWithResume (org.apache.beam.runners.spark.translation.streaming.ResumeFromCheckpointStreamingTest) failed

failed 3 times, but not likely related to this change. Checked out the PR and the test passed locally:

Test	Duration	Result
testWithResume	19.628s	passed