feat: allow external services to be authable

[kylecarbs]

Allows external services to be authenticated with Coder via "Create Workspace".

Shortlist of changes:

1. Renamed environment variables from CODER_GITAUTH_ to CODER_EXTERNAL_AUTH_ (this is backwards compatible).
2. New DISPLAY_NAME and DISPLAY_ICON properties.
3. Newew TYPE=oidc to enable any OpenID Connect provider to work with external auth.

Future improvements:

1. Add a sub-command to the Coder CLI to dynamically obtain the access token from a provider. e.g. coder external-auth refresh --type=github, coder external-auth refresh --id=github, coder external-auth refresh --match="https://github.com"
2. Add coder_external_auth Terraform data source to replace coder_git_auth. Allow for matching based on type and regex so Modules can make use of auth 👀.

[kylecarbs]

@bpmct please review the docs on this change.

We also need to add a permanent redirect for coder.com from /docs/v2/admin/git-auth to /docs/v2/admin/external-auth.

[BrunoQuaresma]

FE code looks good but there are a few Storybook stories broken

[kylecarbs]

Fixed! Good call on checking the Storybook.

[github-actions]

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✔️ PR 9996 Updated successfully.
🚀 Access the credentials here.

cc: @kylecarbs