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 files 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:

[VaggelisD]

Hey @RedZapdos123, thank you for your contribution, leaving a few comments:

[VaggelisD]

One more comment and should be good to go I believe:

[VaggelisD]

• We can pull these upwards to avoid duplicating them as well
• We should avoid doing self.func here, looks a bit hacky given that this is an option list and not a function call

I'd try to rewrite this more like the following (may need minor fixes):

Suggested change

	this = self.sql(expression, "this")
	overwrite = " OVERWRITE" if expression.args.get("overwrite") else ""
	files = self.sql(expression, "files")
	if files:
	files = f"FILES {self.wrap(files)}"
	this = f" {this}" overwrite else f" INTO TABLE {this}"
	return f"LOAD DATA{overwrite}{this} FROM {files_sql}"
	local = " LOCAL" if expression.args.get("local") else ""
	inpath = f" INPATH {self.sql(expression, 'inpath')}"
	this = f" INTO TABLE {this}"