feat: support generic OAuth IDPs

With these changes, the resources and datasources zitadel_idp_oauth and zitadel_org_idp_oauth are added.

Closes #172

[x] Short description of the feature/issue is added in the pr description
[x] PR is linked to the corresponding user story
[x] Acceptance criteria are met
[x] All open todos and follow ups are defined in a new ticket and justified
[x] Deviations from the acceptance criteria and design are agreed with the PO and documented.
[x] No debug or dead code
[x] My code has no repetitions
[x] All non-functional requirements are met
[x] The generic lifecycle acceptance test passes for affected resources.
[x] Examples are up-to-date and meaningful. The provider version is incremented.
[x] Docs are generated.
[x] Code is generated where possible.

zitadel / terraform-provider-zitadel