Chore(optimizer)!: Annotate types for BigQuery's STRING_AGG function

[vchan]

BigQuery's STRING_AGG function accepts STRING or BYTES and returns the corresponding type. It also supports optional aggregate clauses like LIMIT, ORDER BY, and DISTINCT. This PR unwraps these expressions in order to annotate types.

BigQuery documentation on its STRING_AGG function: https://docs.cloud.google.com/bigquery/docs/reference/standard-sql/aggregate_functions#string_agg

[VaggelisD]

Looks like there was a collision with #6463! Initiated a discussion offline for us to solve this moving forward.

For reference, my comment on the other PR would be helpful here too

[georgesittas]

Closing this in favor of Brad's PR.