feat: Nullable enums

[Jille]

For every enum X, generate a type NullX which is a struct with X and
Valid bool.

issue #1097

Remaining work:

• Get agreement from @kyleconroy
• Discuss whether to put in effort to skip generation of NullX when it's unused.
• Discuss whether the backwards compatibility break (because some inputs/outputs will become NullX) is a problem
• Test with pgx
• Test with database/sql postgres
• Test with database/sql mysql

[Jille]

Hmm, looks like this does the same thing as #1485. Let's see if that PR gets merged and we can drop this one at that point.

@SnoozeThis #1485 <= 1 month

[SnoozeThis]

(https://snoozeth.is/Yoys1jlx5c8) I will wait until #1485 is merged and then add a comment (until at most Thu, 02 Jun 2022 17:34:48 CEST).

[SnoozeThis]

The deadline passed while snoozing.

[kyleconroy]

Closing as we merged #1485