Add columns.add RPC function

[mathemancer]

Fixes #3585

Exposes column adding function columns.add at the RPC endpoint.

Technical details

The new RPC function is documented, and no further explanation should be necessary. If it's not clear enough, request a change to this PR, since we should then improve that documentation.

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[Anish9901]

We should probably document the behavior when column_data_list is set to [{}](null)

[Anish9901]

Looks good to me @mathemancer, I've requested a small documentation change. Please merge the PR once it is addressed.