extensibility: add hooks (plugins, discovery, sdk)

This adds a lightweight extensibility mechanism to OPA: hooks. Loosely
modelled on what franz-go supports (see refs below).

We're starting with a configuration hook. It allows us to inspect or
alter the configuration of OPA after...

1. the config is read and parsed: OnConfig
2. a discovery bundle is processed: OnConfigDiscovery

References:
- franz-go: https://pkg.go.dev/github.com/twmb/franz-go/pkg/kgo#Hook

To follow:
- more hooks where they are useful
- runtime support for hooks, wiring them into the proper other places

Signed-off-by: Stephan Renatus <stephan@styra.com>