Started work on a pool manager

[fabianfett]

This is a very early idea dump of a pool manager.

[codecov]

Codecov Report

❌ Patch coverage is 47.85992% with 134 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.46%. Comparing base (66c7cc4) to head (afe2e6d).

Files with missing lines	Patch %	Lines
...s/ConnectionPoolModule/ConnectionPoolManager.swift	0.00%	74 Missing ⚠️
Sources/PostgresNIO/Pool/PostgresClient.swift	64.77%	31 Missing ⚠️
Sources/ConnectionPoolModule/ConnectionPool.swift	62.06%	11 Missing ⚠️
...urces/ConnectionPoolModule/ConnectionRequest.swift	0.00%	9 Missing ⚠️
Sources/ConnectionPoolTestUtils/MockExecutor.swift	25.00%	3 Missing ⚠️
...s/PostgresNIO/Pool/PostgresConnectionRequest.swift	90.00%	3 Missing ⚠️
.../ConnectionPoolModule/ConnectionPoolExecutor.swift	33.33%	2 Missing ⚠️
...onnectionPoolTestUtils/MockConnectionFactory.swift	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #551      +/-   ##
==========================================
- Coverage   76.31%   75.46%   -0.85%     
==========================================
  Files         135      139       +4     
  Lines       10207    10411     +204     
==========================================
+ Hits         7789     7857      +68     
- Misses       2418     2554     +136     

Files with missing lines	Coverage Δ
...s/ConnectionPoolModule/ConnectionIDGenerator.swift	100.00% <ø> (ø)
...nPoolModule/PoolStateMachine+ConnectionGroup.swift	89.94% <100.00%> (-1.06%)	⬇️
...urces/ConnectionPoolTestUtils/MockConnection.swift	96.42% <100.00%> (-3.58%)	⬇️
Sources/ConnectionPoolTestUtils/MockRequest.swift	88.88% <ø> (ø)
Sources/PostgresNIO/Pool/ConnectionFactory.swift	83.48% <100.00%> (ø)
...onnectionPoolTestUtils/MockConnectionFactory.swift	90.19% <92.85%> (ø)
.../ConnectionPoolModule/ConnectionPoolExecutor.swift	33.33% <33.33%> (ø)
Sources/ConnectionPoolTestUtils/MockExecutor.swift	25.00% <25.00%> (ø)
...s/PostgresNIO/Pool/PostgresConnectionRequest.swift	90.00% <90.00%> (ø)
...urces/ConnectionPoolModule/ConnectionRequest.swift	42.85% <0.00%> (-11.69%)	⬇️
... and 3 more

... and 2 files with indirect coverage changes

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