feature: Document AuthBridge in-process plugin support better

[esnible]

Feature Description

Users encountering our README without knowing about the feature flag tags of AuthBridge find it confusing.

Proposed Solution

Rework the README to document our policy, which is:

• We support all of the AuthBridge plugins
• Developers writing new AuthBridge plugins can use go.features to test their code locally, without needing container images
• Developers adding new AuthBridge plugins will need to let us know to manually add them for all when the plugin is completed
• Developers can test container images locally using a flag instead of in-process

Want to contribute?

• I would like to work on this issue.

Additional Context

No response

[esnible]

Closed through #71