Add support for SNI plugin

[maskit]

This enables writing SNI actions as plugins.

The implementation is experimental. We'd have to introduce new TS APIs to do various things on the plugins, but that would be next step.

To use SNI plugins, you need to use plugins keyword on sni.yaml. For example:

sni:
- fqdn: example.com
  http2: on
  plugins:
    - path: my_sni_action.so
      parameters: test

This is a draft. It does not have code for config reload and cleanup.