fix(bigquery)!: Disable STRING_AGG separator canonicalization

[VaggelisD]

Fixes #6392

Canonicalizing STRING_AGG with a string literal comma separator is not correct in case the expression is of BYTES type:

bq> SELECT STRING_AGG(a) FROM UNNEST([b'foo', b'bar', b'baz']) AS a
Zm9vLGJhcixiYXo= -- BASE64 decoding: foo,bar,baz

bq> SELECT STRING_AGG(a, ',') FROM UNNEST([b'foo', b'bar', b'baz']) AS a
Zm9vLGJhcixiYXo= -- BASE64 decoding: foo,bar,baz

No matching signature for aggregate function STRING_AGG Argument types: BYTES, STRING

Docs

BQ STRING_AGG

[erindru]

/integration-tests