feature/derive db enum2

[demhadais]

• implement at least postgres deserialization and serialization of an enum type
• basically complete the implementation
• partial commit
• turns out the function was done. just remove the unnecessary lines of code
• rename the derive-macro to something a bit simpler. I'm not sure if this is good though. Also, write documentation. Also, write two simple tests
• add a test that actually tries inserting the color into the database
• remove failing test
• give parsed_backends set explicit capacity
• implement FromSql and ToSql for generated types, not Text
• fix syntax for repeating elements
• add as_expression and fromsqlrow impls
• try to make the test work by using the derive correctly
• fix test helper code
• add sql_type to allowed parsing. I anticipate this will have to be reworked a bit
• fix clippy and formatting recommendations
• try to rely on the already implemented check-for-backend functionality to make it work
• revert the rust tooclhain to fix the conflict
• remove custom attribute and just use the model parsing because it's there
• gate the enum test behind a feature to make it work

[demhadais]

@weiznich Sorry this took so long, but just pinging you because I had a draft PR that I closed, and I haven't worked on this in a long time. Anyways, this implements the first step of #4759.

The tests are super basic, and I'm happy to add more, but I couldn't think of any meaningful tests that aren't redundant with other tests in the codebase.

Thanks!