Update OnCall client setup to support service account auth

[matiasb]

Related to https://github.com/grafana/oncall-private/issues/2826

Requires grafana/amixr-api-go-client#24 (and a client release + updating dep here)

[github-actions]

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[joeyorlando]

overall LGTM. Should we update the provider documentation on service account vs oncall api token auth here? Do you think this warrants a major-version bump of the provider?

[matiasb]

overall LGTM. Should we update the provider documentation on service account vs oncall api token auth here? Do you think this warrants a major-version bump of the provider?

Yeah, I will be updating some docs here before merging (still need to release the updated API client and push some changes here, besides I want to do some additional testing too). About the provider version, not sure who/how that is decided (in any case we are not breaking any compatibility just extending the auth support).

Also I would like to update some other docs I found re OnCall and Terraform once I confirm everything working as expected.