Make BigQuery typeless STRUCTs Expressions

[tunetheweb]

Brief summary of the change made

Fixes #1913

This makes BigQiery STRUCT statements full expressions so they can be used everywhere, rather than an add-on that can only be used in some places.

Are there any other side effects of this change that we should be aware of?

Adds the struct type to ANSI (as Nothing) to allow it to be part of Expression C and then overridden in BigQuery.
Cleans up some other references to TypelessStructSegment as no longer needs to be directly referenced as now can be referenced through an Expression.

Pull Request checklist

• Please confirm you have completed any of the necessary steps below.

• Included test cases to demonstrate any code changes, which may be one or more of the following:

  • .yml rule test cases in test/fixtures/rules/std_rule_cases.
  • .sql/.yml parser test cases in test/fixtures/dialects (note YML files can be auto generated with tox -e generate-fixture-yml).
  • Full autofix test cases in test/fixtures/linter/autofix.
  • Other.

• Added appropriate documentation for the change.

• Created GitHub issues for any relevant followup/future enhancements if appropriate.

[codecov]

Codecov Report

Merging #2435 (1eb5b52) into main (6ccaaa9) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #2435   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          162       162           
  Lines        11571     11571           
=========================================
  Hits         11571     11571           

Impacted Files	Coverage Δ
src/sqlfluff/dialects/dialect_ansi.py	100.00% <100.00%> (ø)
src/sqlfluff/dialects/dialect_bigquery.py	100.00% <100.00%> (ø)

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[barrywhart]

Looks good. Very nice cleanup!