Introduce EJBCA UpstreamAuthority plugin for SPIRE Server

[m8rmclaren]

Pull Request check list

• Commit conforms to CONTRIBUTING.md?
• Proper tests/regressions included?
• Documentation updated?

Affected functionality

This PR introduces EJBCA as a Server UpstreamAuthority plugin. This plugin uses a connected EJBCA to issue intermediate certificates for the SPIRE server.

Per #4163, the EJBCA UpstreamAuthority plugin is compatible with EJBCA Community.

Description of change

• Adds EJBCA as a built-in UpstreamAuthority plugin.
• Adds the EJBCA Go Client SDK as a required dependency for communicating with the EJBCA REST API.
• Updates the golang.org/x/oauth2 to v0.21.0 required when importing the EJBCA Go Client SDK.

Which issue this PR fixes

[amartinezfayo]

Thank you @m8rmclaren for this contribution!
While we are waiting for the integration tests to complete the review, I just wanted to provide a couple of comments.

[m8rmclaren]

Hey everyone, here's the result of the EJBCA UpstreamAuthority integration test running on my Mac:

[m8rmclaren]

Hi @amartinezfayo -

I made the recommended changes to ejbca.Config and resolved what seemed like the issue in the EJBCA integration test. Same as before, the test succeeds on my Mac.

[amartinezfayo]

Hi @m8rmclaren, just checking in here. Do you need any help or have questions about the latests comments?

[m8rmclaren]

Hi @amartinezfayo!

I believe I've resolved the suggestions from your review, with the most important change being the removal of OAuth as an auth method. I also resolved the issue in the EJBCA integration test, and verified that the GitHub Actions run in my fork.