Clarify `data_sources.update_method`

[maxachis]

Context

The update_method attribute of data_sources currently possesses one of three values (as well as null):

• "Insert"
• "Overwrite"
• "No updates"

It is unclear how open-ended update methods are, and if we expect others. If we don't expect it to be open ended, we may want to limit the number of possible inputs for ease of use

Requirements

• Determine if update_method is comprehensive, and if we want to have update methods be specified by selecting from one of several options, or by inputing open-ended text.

Tests

• Not applicable

Docs

• Not applicable.

Open questions

• See above.

[josh-chamberlain]

@maxachis Let's call this one a select with these 3 options. That's how it is in Airtable, which I think we can match.

[maxachis]

Perfect! As it turns out, I already have an enum encoding that, so we're all good here!