In the first PR for file source V2, there was a rule for falling back Orc V2 table to OrcFileFormat: https://github.com/apache/spark/pull/23383/files#diff-57e8244b6964e4f84345357a188421d5R34
As we are migrating more file sources to data source V2, we should make the rule more generic. This PR proposes to:
1. Rename the rule `FallbackOrcDataSourceV2 ` to `FallBackFileSourceV2`.The name is more generic. And we use "fall back" as verb, instead of "fallback".
2. Rename the method `fallBackFileFormat` in `FileDataSourceV2` to `fallbackFileFormat`. Here we should use "fallback" as noun, while "fall back" is verb.
3. Add new method `fallbackFileFormat` in `FileTable`. This is for falling back to V1 in rule `FallbackOrcDataSourceV2 `.