Async/await connection pool source

[adam-fowler]

Add support for async/await connections

• HBAsyncConnectionSource: for generating Connections using async
• HBAsyncConnection: which includes an async close function

[codecov]

Codecov Report

Merging #121 (fa8c439) into main (861d346) will decrease coverage by 0.32%.
The diff coverage is 8.33%.

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
- Coverage   80.46%   80.13%   -0.33%     
==========================================
  Files          72       72              
  Lines        3061     3071      +10     
==========================================
- Hits         2463     2461       -2     
- Misses        598      610      +12     

Impacted Files	Coverage Δ
...gbird/AsyncAwaitSupport/ConnectionPool+async.swift	0.00% <0.00%> (ø)
...es/Hummingbird/ConnectionPool/ConnectionPool.swift	91.89% <50.00%> (ø)
Sources/HummingbirdJobs/MemoryJobQueue.swift	93.54% <0.00%> (-6.46%)	⬇️

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