source-postgres: use flat arrays rather than array objects w/ dimensions

[willdonnelly]

Backstory:

Currently source-postgres captures array columns as a JSON object with dimensions and elements properties, each of which is an array. This is because PostgreSQL arrays are inherently multidimensional, and we're trying to preserve them as faithfully as possible.

(The JSON schema for a recursive "array whose elements can each be type X or another nested array" type is so incredibly terrible that it was never a realistic option, so "a flat array of values plus a list of array dimensions" was the best we could do to represent the original value faithfully)

But it turns out that nobody ever wants that, they just want simple, boring one-dimensional arrays of values to translate into a JSON array of equivalent values. So we should do that.

Plan:

1. Add a feature flag flattened_arrays.
   a. When flattened_arrays is set, value translation will return just the elements array instead of the array-with-dimensions object.
   b. When flattened_arrays is set, discovered schemas will of course reflect that change in behavior.
2. Set no_flattened_arrays on all source-postgres captures in production.
3. Merge a followup PR which toggles the default for new captures.