Add support for authenticating with OpenID Connect

[geofflamrock]

Is your feature request related to a problem? Please describe.
Octopus now has support for authenticating to the API using OpenID Connect (OIDC), replacing the need to provision and manage API keys for automations from external systems.

This involves configuring an OIDC identity on a service account with details on which tokens can be trusted from an OIDC provider (for example GitHub Actions), exchanging an ID token from this provider for a short-lived Octopus access token which can then be used to authenticate to the Octopus API.

Describe the solution you'd like
Add support for configuring the Octopus terraform provider to use OIDC. We expect this could look something like:

• Configuring the service account id to use within the provider config
• Performing the token exchange outside of the terraform provider to obtain an access token and use this within the provider, either as a CLI parameter or from an environment variable.
  • An alternative to this might be to provide the ID token into the terraform provider somehow and have it do the exchange.

[Richard-de-Vries]

This would be something i would be interested in. Our case would be to get the authentication token in a github actions pipeline, and passing this to the octopus terraform provider.