Add columns.patch RPC function

[mathemancer]

Fixes #3584

Adds a columns.patch RPC function. The function is well-documented, and should be usable/understandable from that documentation.

Technical details

This still uses some transformation scaffolding to munge the representation of columns in the API into the form used in the database. After discussing with @pavish , we've decided to leave that scaffolding in, and we'll probably reassess the representation of Columns in the API at a later date.

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• I added tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
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  visible errors.

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[Anish9901]

Looks great!