feat: multiple 'on' columns in upsert

[thegreymatter]

Is your feature request related to a problem?

I'm missing the ability to do upsert on multiple columns. Currently On param support a single string - i.e a single column name.

What is the motivation behind your request?

upserting on multiple key for example tenant_id, entity_id

Describe the solution you'd like

support to optionally put list of Column object or strings.

What version of ibis are you running?

12.0.0

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

BigQuery
DuckDB

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