Update Go READMEs with Slack badge and new ecosystem projects

[akshayjshah]

We should make another pass over the READMEs for our Go projects and make a few improvements:

1. Add a Slack badge (like the authn README's). For now, we can link to the Buf Slack. If the CNCF application goes through, we may want to switch to the CNCF Slack instead.
2. Add the CORS and authn packages to the ecosystem section.

@chrispine none of this is urgent, just want to get it on your radar.