feat: [contentwarehouse] A new message FoundationModelTuningOptions…

… is added (#5243)

* feat: A new message `FoundationModelTuningOptions` is added
feat: A new field `foundation_model_tuning_options` is added to message `.google.cloud.documentai.v1.TrainProcessorVersionRequest`
docs: updated comments

PiperOrigin-RevId: 625415406

Source-Link: googleapis/googleapis@caa099d

Source-Link: googleapis/googleapis-gen@9168448
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