fix(bigquery): support LOAD DATA FROM FILES syntax

[RedZapdos123]

Summary:

• add BigQuery parser support for LOAD DATA ... FROM FILES(...) syntax used in BigQuery docs
• keep existing LOAD DATA ... INPATH ... parsing behavior by falling back to the base parser when FROM FILES is not matched
• make LoadData.inpath optional and add optional iles payload for BigQuery form
• add BigQuery generator override to emit LOAD DATA [OVERWRITE|INTO TABLE ...] FROM FILES(...)
• add BigQuery regression tests for both OVERWRITE ... FROM FILES(...) and INTO TABLE ... FROM FILES(...)

Closes #7480.

Validation:

• Reproduced issue before fix: ParseError: Required keyword: 'inpath' missing
• python -m pytest -q tests/dialects/test_bigquery.py -vv
• python -m pytest -q
• Direct parse reproduction now succeeds, and returns LoadData

The validations screenshots of the tests run, locally on WSL: