BigQuery: Add to_standard_sql() method to SchemaField

[plamut]

Closes #8494.

This PR adds a method to convert a legacy table field instance to the new StandardSqlField type.

How to test

Verify that the types are correctly mapped from the legacy to standard SQL, and that the new method does what it is supposed to do.

[plamut]

False positive here, flake8 thinks that is about comparing types and wants to enforce isinstance() - but that would not work, as we actually compare integers here.

[plamut]

Support for standard SQL type names and simple arrays added. Still need to add support for arrays containing STRUCTs, do not merge yet.

[tswast]

Looks good. Thanks for adding tests for arrays of structs.

This reminds me that I wrote some similar code recently at

google-cloud-python/bigquery/google/cloud/bigquery/_pandas_helpers.py

Line 116 in 8c8e360

def bq_to_arrow_data_type(field):

Would it make any sense to share any of that code here?

[plamut]

@tswast You mean sharing the conversion logic or the test for it?

Since the "target results" differ, there are differences between the two implementations, and they are both reasonably short, too, thus we might not actually gain that much by refactoring.

(but of course, if you see something "obvious" that would represent a good opportunity, we can add this improvement in another PR)

[tswast]

Agreed. Not obvious to me right now how to share code without it getting more complex.