feat: add support for OIDC bearer tokens to auth TFP against Octopus API

[mjhilton]

This PR adds some small improvements to the auth experience of the Terraform Provider:

• Initialises the internal Go Client with an identifier of "TerraformProvider", which allows us to disambiguate requests are coming from use of the Terraform Provider versus other Go Client consumers in API logs
• Adds fall-through support for the more standard OCTOPUS_API_KEY environment variable, which gets set by things like the GitHub Actions Octopus Login action when using an API Key. The existing OCTOPUS_APIKEY environment variable is still preferred when present.
• Adds support for OIDC Bearer Tokens to be used, either directly configured on the provider, or with the OCTOPUS_ACCESS_TOKEN environment variables (which the GitHub Actions Octopus login action sets when using OIDC auth)
• Updates the docs to detail the additions/changes to the auth approach, and expand upon explanation of environment variable precedence and interaction with pipeline steps

Fixes #579
Internal story [sc-91345]

[mjhilton]

I've done some testing in GitHub Actions and confirmed that:

• The UserAgent is now distinguishable in request logging, with the string TerraformProvider inserted as expected
• The OIDC integration works out-of-the-box with the GitHub Login action, based on the conventional environment variables that action sets (OCTOPUS_URL; OCTOPUS_ACCESS_TOKEN)
• API Key auth is preferenced over OIDC when both are available (for back-compat)
• API Key auth now pulls from either OCTOPUS_APIKEY or OCTOPUS_API_KEY env variables, and is as such now compatible with the GitHub Login action when it uses API Key mode

[domenicsim1]

Nice!
Good stuff on the Docs 👍

[domenicsim1]

👍