OctopusID doesn't support authorization code flow

[N-lson]

Prerequisites

• I have searched open and closed issues to make sure it isn't already requested
• I have discussed this in #feature-discussion or #backlog and the consensus is that this is something we plan on adding in the near future
• I have written a descriptive issue title
• I have linked the original source of this feature request
• I have labelled the value stream (area/core, area/steps, ...)
• I have added the kind/enhancement label

The enhancement

What is the problem this solves or benefit it gives

Since we've upgraded the base OpenIDConnect auth provider to use authorization code flow + PKCE, all the OIDC providers are now able to use this flow except for OctopusID. This is because our implementation of OAuth in OctopusID doesn't currently support auth code flow.

Proposed solution

Add authorization code flow support to OctopusID. A starting point would be OctopusInstanceAuthorizeController in Octofront. Once done, the OIDC provider can be updated to remove the check for OctopusID.

Make sure the following clients aren't broken:

• Octofront
• Commercial billing portal
• Insights

Links

Internal slack discussion: https://octopusdeploy.slack.com/archives/C01J6U3MHJ4/p1650585004013579