add opa intergration to support complex silence creation control

[LeoQuote]

Currently , karma has a simple silence acl which is documented well -> https://github.com/prymitive/karma/blob/main/docs/ACLs.md

but in reality this is sometimes not enough.

opa can make it possible to take everything possible into consider, including:

1. user info from jwt header, not just username, email, groups, but all the info in jwt token.
2. alerts that affected , maybe refuse a silence that would affects over 100 alerts?

this could be done with envoy and envoy opa plugin, but embed this in karma could be better