Support field names in BigQuery STRUCT

[gomesfernandes]

I want to parse BigQuery queries that assign field names in their STRUCTs.
Example:

SELECT
    STRUCT(
        CONCAT(user.first_name, ' ', user.last_name) AS full_name,
        user.email AS email,
    ) AS viewer
FROM users

This corresponds to the typeless struct syntax mentioned in the BigQuery documentation, and which is as follows:
STRUCT( expr1 [AS field_name] [, ... ])

Note that without the aliases, the query is parsed successfully:

SELECT
    STRUCT(
        CONCAT(user.first_name, ' ', user.last_name),
        user.email
    ) AS viewer
FROM users

It seems there is already an ongoing PR related to BigQuery STRUCTs, but the description does not mention support for field names/aliases.

[alamb]

Perhaps @togami2864 has some time to revive #817, and if not perhaps someone else can pick it up