Add scala-xml module

[bplommer]

[codecov[bot]]

Codecov Report

Merging #664 (89426bb) into main (14c756d) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #664      +/-   ##
==========================================
+ Coverage   85.02%   85.03%   +0.01%     
==========================================
  Files         124      125       +1     
  Lines        1629     1631       +2     
  Branches       38       38              
==========================================
+ Hits         1385     1387       +2     
  Misses        244      244              

Impacted Files	Coverage Δ
modules/scalaxml/src/main/scala/XmlCodecs.scala	100.00% <100.00%> (ø)

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[bplommer]

Needs mima-wrangling to exclude the new module.

[armanbilge]

tlVersionIntroduced := List("2.12", "2.13", "3").map(_ -> "skunk version this will be added").toMap

[bplommer]

Note on testing - I've copied the pattern of the Circe tests but they aren't really adequate (they'll still pass if you change the Postgres types of the codecs.) I've done some manual end-to-end testing but we don't have any automated tests that these codecs actually work correctly with Postgres.