Feat(BigQuery): Add support for column descriptions in nested fields

[themisvaltinos]

This PR introduces support for column descriptions in nested types in BigQuery, closes #3854

The syntax follows BigQuery's conventions for nested and repeated columns, using dots to indicate the appropriate level of nesting:

MODEL (
  name schema.record_model,
  column_descriptions(
    id = "Unique identifier for the event",
    repeated_record.sub_repeated_record.sub_field = "Sub-field within nested repeated record",
    record.nested_record.field = "Field within nested record",
    record.nested_record.array = "Array of integers",
    repeated_record.sub_repeated_record = "Nested repeated record",
    record.nested_record = "Nested record"
  )
);

SELECT
  id,
  [STRUCT([STRUCT(sub_field AS sub_field)] AS sub_repeated_record)] AS repeated_record,
  STRUCT(
    STRUCT([2, 3] AS array, field AS field) AS nested_record
  ) AS record
FROM
  schema.seed_model;

The column definitions are built using the helper method _build_column_def, which is applied recursively in the BigQuery adapter to handle the RECORD and REPEATED RECORD columns. For views which are registered in _create_column_comments we traverse the fields and apply comments at each appropriate level.

[georgesittas]

A couple more comments, but LGTM otherwise.