handle simple enum types

[clux]

Currently, I think we will error out, or omit enum parts of the schema.

It would be helpful to either find a good / official CRD from somewhere that relies on enums, or even better, a POC PR that showcases how to parse them.

[alex-hunt-materialize]

Currently, I think we will error out, or omit enum parts of the schema.

Current behavior is to omit the enum part, and just treat it (in this example) as a simple String.

operator:
  enum:
  - In
  - NotIn
  - Exists
  - DoesNotExist
  type: string

becomes

pub operator: String,

A complete example can be found in the Linkerd 2.11 ServerAuthorization and Server CRDs (at least, as generated by their helm chart). I've saved the generated one in a gist, since I can't find an official download of a pre-generated CRD.
https://gist.github.com/alex-hunt-materialize/2743b1e2e58a49c4df0a11ecb39f46ab

These CRDs actually have several issues with the current generation, but I'll make separate issues for those.

[clux]

Handling simple enum's like this turns out not to be particularly difficult. Provided there are no other cases that can set type to something else or leave it unset, this will be handled by #65. The more complex enums using oneOf is going to be more complicated (and will open some follow-ups), but will try to use the ServiceMonitor as acceptance criteria for this issue as it only has a simple enum.