Add support for LISTEN and NOTIFY

[johnbona]

I'm not all that comfortable with libpq so give this extra scrutiny. 🙊

I used this example heavily to help me determine how to receive notifications using libpq. I also purposefully broke convention and used a closure in listen instead of creating a onNotification property off Database since a database could be listening to multiple channels simultaneously.

Couple of things that I wanted to point out for feedback:

• What to do with errors caught in listen since the closure in try background can't throw? Using fatalError right now but not sure if that's the correct approach.
• The name of the Notification struct. It's not really meant to be used outside the listen closure so it's maybe okay to keep named as is even though it collides with Foundation's own Notification.

[codecov-io]

Codecov Report

Merging #28 into master will increase coverage by 0.01%. The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   93.36%   93.37%   +0.01%     
==========================================
  Files          11       12       +1     
  Lines        1748     1812      +64     
==========================================
+ Hits         1632     1692      +60     
- Misses        116      120       +4

Impacted Files	Coverage Δ
Sources/PostgreSQL/Notification.swift	100% <100%> (ø)
Tests/PostgreSQLTests/PostgreSQLTests.swift	95.72% <100%> (+0.22%)	:white_check_mark:
Sources/PostgreSQL/Database.swift	89.13% <85.71%> (-1.5%)	:x:

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[johnbona]

Not sure what happened there so recreated the PR. 😅