Started work on a pool manager

[fabianfett]

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

[codecov]

Codecov Report

❌ Patch coverage is 13.35740% with 240 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.48%. Comparing base (4ebc10c) to head (3dc6c2f).

Files with missing lines	Patch %	Lines
Sources/PostgresNIO/Pool/PostgresClient.swift	0.00%	93 Missing ⚠️
...s/ConnectionPoolModule/ConnectionPoolManager.swift	0.00%	82 Missing ⚠️
...s/PostgresNIO/Pool/PostgresConnectionRequest.swift	0.00%	36 Missing ⚠️
Sources/ConnectionPoolModule/ConnectionPool.swift	62.06%	11 Missing ⚠️
...urces/ConnectionPoolModule/ConnectionRequest.swift	0.00%	11 Missing ⚠️
Sources/ConnectionPoolTestUtils/MockExecutor.swift	25.00%	3 Missing ⚠️
.../ConnectionPoolModule/ConnectionPoolExecutor.swift	33.33%	2 Missing ⚠️
...onnectionPoolTestUtils/MockConnectionFactory.swift	92.85%	1 Missing ⚠️
Sources/PostgresNIO/Pool/ConnectionFactory.swift	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #551      +/-   ##
==========================================
- Coverage   62.60%   61.48%   -1.13%     
==========================================
  Files         132      136       +4     
  Lines       10424    10645     +221     
==========================================
+ Hits         6526     6545      +19     
- Misses       3898     4100     +202     

Files with missing lines	Coverage Δ
...s/ConnectionPoolModule/ConnectionIDGenerator.swift	100.00% <ø> (ø)
...urces/ConnectionPoolTestUtils/MockConnection.swift	96.92% <100.00%> (+0.14%)	⬆️
Sources/ConnectionPoolTestUtils/MockRequest.swift	80.00% <ø> (ø)
...onnectionPoolTestUtils/MockConnectionFactory.swift	91.52% <92.85%> (ø)
Sources/PostgresNIO/Pool/ConnectionFactory.swift	0.00% <0.00%> (ø)
.../ConnectionPoolModule/ConnectionPoolExecutor.swift	33.33% <33.33%> (ø)
Sources/ConnectionPoolTestUtils/MockExecutor.swift	25.00% <25.00%> (ø)
Sources/ConnectionPoolModule/ConnectionPool.swift	91.17% <62.06%> (-3.63%)	⬇️
...urces/ConnectionPoolModule/ConnectionRequest.swift	45.83% <0.00%> (-13.63%)	⬇️
...s/PostgresNIO/Pool/PostgresConnectionRequest.swift	0.00% <0.00%> (ø)
... and 2 more

... and 1 file with indirect coverage changes

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