feature: no compilation rule for TimestampDiff operation in bigquery

[printsg]

What happened?

Raising the same issue as in archived repository

ibis-project/ibis-bigquery#70

ibis.to_sql(expr.mutate(dt = _.timestamp.sub(_.timestamp)))

Results in error:
ibis.common.exceptions.OperationNotDefinedError: Compilation rule for 'TimestampDiff' operation is not defined

What version of ibis are you using?

11.0.0

What backend(s) are you using, if any?

BigQuery

Relevant log output

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