Support plugin-specific rule links

Feature

It looks like the buf lsp hardcodes annotation rule links to https://buf.build/docs/lint/rules/#<ruleID> which doesn't make sense for plugin-provided annotations. Ideally the annotation serialization format supports providing both a "sub rule" and the link to the rule documentation. For example, here is the output for a wrapper plugin I wrote for AIP's api-linter, but I'm unable to link to the real rule because it's hidden behind the bufplugin protocol.

I'm happy to try and contribute a fix for this, but wanted to make sure this is something folks wanted to support.