Add support for UDS and existing Channels

[gwynne]

Extends PostgresConnection to enable connecting to Postgres servers over UNIX domain sockets or using a preexisting Channel.

In the process, most of the existing structure of PostgresConnection.Configuration has been deprecated in favor of new APIs with more options and cleaner design. The deprecation messages provide instructions on migrating existing code. List of deprecations and their replacements (all listed APIs should be assumed to have a PostgresConnection. prefix):

• Configuration.Connection and .connection -> Configuration.host, .port, .unixDomainSocket, and .establishedChannel properties, and respective initializers.
• Configuration.Authentication and .authentication -> Configuration.username, .password, and .database properties.
• New Configuration.Options structure and .options property, containing the connectTimeout and requireBackendKeyData properties previously found on Configuration.Connection.

Note: The integration tests for UNIX domain sockets will be skipped unless POSTGRES_SOCKET is set to a socket path in the test environment. See the updated test.yml for examples of how to do so correctly.

Closes #259
Closes #268

[gwynne]

/cc @MahdiBM @tkrajacic

[gwynne]

This PR is intended to subsume both #302 and #314, lmk if there's anything missing.

[0xTim]

Resolves #259

[gwynne]

CI failure on nightly-main is tracked by swiftlang/swift#65064

[0xTim]

LGTM - only thing to mention that I couldn't quite tell is that it looks like all the tests have been updated to use the new APIs. We should probably have a test just to ensure the deprecated stuff still works

[codecov-commenter]

Codecov Report

Merging #335 (e8ad7df) into main (c996d62) will decrease coverage by 0.23%.
The diff coverage is 32.16%.

❗ Current head e8ad7df differs from pull request most recent head f8ae7a2. Consider uploading reports for the commit f8ae7a2 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #335      +/-   ##
==========================================
- Coverage   41.27%   41.04%   -0.23%     
==========================================
  Files         115      117       +2     
  Lines        9572     9657      +85     
==========================================
+ Hits         3951     3964      +13     
- Misses       5621     5693      +72     

Impacted Files	Coverage Δ
.../PostgresConnection+Configuration+Deprecated.swift	0.00% <0.00%> (ø)
...es/PostgresNIO/New/Data/UUID+PostgresCodable.swift	96.96% <ø> (ø)
Sources/PostgresNIO/New/PostgresCodable.swift	93.87% <ø> (ø)
...es/PostgresNIO/Connection/PostgresConnection.swift	12.03% <22.22%> (-4.60%)	⬇️
.../Connection/PostgresConnection+Configuration.swift	44.44% <44.44%> (ø)
...urces/PostgresNIO/New/PostgresChannelHandler.swift	62.06% <100.00%> (+0.92%)	⬆️

[fabianfett]

Looks way better.

[fabianfett]

LOVELY! Nits

[fabianfett]

Lovely! Thanks so much!