Add `singleton` POST operations

[joshlang232]

New Feature

• For singleton resources, it seems plausible to want to be able to create a single instance of the resource. Likely this would never be used, but it could theoretically get used once (or more if people are allowed to delete it too). I imagine we may want to have 409 error codes more natively added for singleton resources, but that's more work than I am willing to look into doing.
• For singleton subresources, we have a use case where we want to create a subresource after creating the main resource. That being said that, it is a bit strange to do a POST call for the singleton subresource and get nothing back (since we do not have an ID attribute), but not sure if it's better to return the entire object after creation or just let people do a GET afterwards.

Example Use Case The Activations Backend team wants to have a singleton subresource that requires a ton of information to create. We could not add a POST on the singleton subresource so we needed write a custom action to do it: https://github.com/LiveRamp/api-specs/pull/464

[ops-github-DU4JOAWE]

This change is 

[njaczko]

Hey @joshlang232, it takes a bit of effort to review Reslang PRs. Sorry we didn't get to this this week, hopefully we can get to it next week.

[joshlang232]

No worries! @njaczko Appreciate the ACK. Let me know if you need anything else from me. Definitely would like to get this merged in within the next week or two before we have actual consumers using the "custom" action in our API

[njaczko]

Thanks for the walkthrough, @joshlang232 . I played with singleton POSTs for a bit and everything I tried worked correctly. LGTM.