fix: return unsupported error for merging schemas in the presence of partition columns

[emcake]

…partition columns

Description

This causes attempts to write to a partitioned table with MergeSchema to fail, as it's not supported by the code.

I took a look at trying to make it work, but there isn't a quick fix. This is because we need a merged schema definition before we start trying to partition by the partition columns, otherwise the newly added columns get dropped. The schema reported for matching in self.arrow_schema_ref also needs to contain the partition columns, and ordering matters in comparing schemas so we need to know the right place to insert them. I think for situtations where flush() has already been called, we also need a function to return a Option<MetaData> action to be applied to manual commits. Finally, re-using a writer in the presence of schema evolution is dangerous, as the original_schema_ref is never updated to match the newly changed schema.

I'd love to follow up with a fix, but in the short term I'd like to just stop others get bitten like I did.

Related Issue(s)

#2468

[github-actions]

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