feat: Adds support for authenticating with access token

[geofflamrock]

This PR adds support for authenticating using an access token as a bearer token in the Authorization header. Access tokens are obtained using OpenID Connect and by nature are short-lived. As such, storing access tokens in the cli configuration (octopus config set) is not supported, and are only read from the OCTOPUS_ACCESS_TOKEN environment variable. This environment variable will be set automatically when using the OctopusDeploy/login action which is currently only released internally.

Note: This PR requires OctopusDeploy/go-octopusdeploy#201 to be completed and a new version of the go-octopusdeploy client to be released before it will build. Update: This PR is merged now.

Note: The OpenID Connect feature is currently under development and may not be available on all Octopus instances at the moment, so this PR doesn't make any changes to the readme for the CLI. Once the feature is released then a follow up PR will add documentation around this.

[johnsimons]

A small comment regarding the init function, but overall seems reasonable to me.
That said, this is the first time I am looking at this code base, so it may be a good idea to ping others who are more familiar with this code base.

[benPearce1]

I would like to see the ApiCredentials struct dropped in favour of an interface with an Api Key implementation and Access Token implementation.
This is actually a change in the client that would impact this PR

[benPearce1]

👍