rules: allow to apply project-specific, class-specific ruleset

[hyangah]

The current implementation uses a single ruleset (internal/rules/static/ruleset.json).
Since gaby can handle multiple GitHub projects (and Gerrit when we start to apply rules to CLs)
and issues in different projects have different rules, we need to adjust internal/rules
to use different rules based on the projects.

Alternatively, we can have a ruleset.json that lists repos info and let LLM figure out.

cc @randall77

[jba]

This would be part of a general config effort (#47).
I think that is not a priority, but if it is I would take it on.

[hyangah]

My motivation behind this is related to #51.

The vscode-go issue tracker has pretty different rules (or no rules) compared to the go issue trackers.