Adds polymorphic relationships for Workspaces, Tokens, and RunTriggers

[brandonc]

This PR adds support for polymorphic fields that already exist in the API to Workspaces (locked-by), AuthenticationTokens (created-by), and RunTriggers (sourceable)

The purpose is to illustrate how the underlying jsonapi feature is used in go-tfe and also to run the upstream PR against a more elaborate test suite.

It uses this dependency PR so that should be merged and released before this PR is merged.

[nfagerlund]

This seems great. I know it's "do not merge," but I'll register my approval for future reference.

If you're really feeling fired up, the one polymorphic relation that I REALLY got bit by lacking in the past was Workspace.LockedBy. I wanted to use it to give a more reliable error description when building out the saved cloud plans apply action in terraform core, but had to give up and give a "usually probably right" error instead.

I think the options for locked-by are user/team/run -- org token can't lock a workspace.

[brandonc]

@nfagerlund What a stroke of luck! Adding LockedBy revealed a bug in the jsonapi decoder when the field was omitted from the payload, which is the case when the workspace is not locked. I also added CHANGELOG entries and will merge this once we have a jsonapi release. Thanks for the suggestion!

[nfagerlund]

🙌🏼!! (I had a hunch aiming it at something I tried to use in anger would turn up something interesting 😅)

[sebasslash]

🚀 -- Awesome work!

[github-actions]

Reminder to the contributor that merged this PR: if your changes have added important functionality or fixed a relevant bug, open a follow-up PR to update CHANGELOG.md with a note on your changes.