L4 module for Postgres: matchers

[metafeather]

This PR adds additional matchers for the Postgres module:

1. Postgres Matcher can now match on the combination of user and database parameters when SSL is disabled
2. New PostgresClient Matcher that checks the application_name parameter when SSL is disabled
3. New PostgresSSL Matcher that can enforce/reject the use of SSL connections, but cannot match on their content

More tests and docs are in progress, but given the discussion in #187 I'd like to share the non-SSL matchers, and discuss further enhancements to the PostgresSSL Matcher

[mholt]

/cc @coolaj86 in case you'd be interested in this

[mholt]

Thanks for updating this. Will probably give it another review soon.

[mholt]

Thanks, this looks quite involved! I'm glad you are doing this 😄

I probably won't be testing it myself, so, I imagine you've tested that this works?

One other question I had is: is it possible to have just 1 matcher? I vaguely understand why Postgres SSL is different from regular SSL (but can we call it TLS?). I just wonder if we could simplify the config a bit, or maybe you can help me understand why they should be different matchers entirely... thanks!