fix(unparser): Fix BigQuery timestamp literal format in SQL unparsing

[sgrebnov]

Which issue does this PR close?

The default Dialect::timestamp_with_tz_to_string uses dt.to_string() which produces timestamps with a space before the TimeZone offset. This causes filter pushdown to fail when unparsing timestamp predicates for BigQuery.

2016-08-06 20:05:00 +00:00 <- invalid for BigQuery:
invalid timestamp: '2016-08-06 20:05:00 +00:00'; while executing the filter on column 'startTime' (query) (sqlstate: [0, 0, 0, 0, 0], vendor_code: -2147483648)

BigQuery rejects this format. Per the BigQuery timestamp docs, the offset must be attached directly to the time:

2016-08-06 20:05:00+00:00 <- valid

What changes are included in this PR?

Following similar to DuckDB pattern/fix override timestamp_with_tz_to_string for BigQueryDialect to produce valid timestamp format

Before (default dt.to_string()):

CAST('2016-08-06 20:05:00 +00:00' AS TIMESTAMP)  -- BigQuery error

After (%:z format):

CAST('2016-08-06 20:05:00+00:00' AS TIMESTAMP)  -- valid BigQuery timestamp

Are these changes tested?

Added unit test and manual e2e test with Google BigQuery instance.

Are there any user-facing changes?

No