Request encoding user info

[adam-fowler]

• Add RequestEncodingContainer class that holds references to request headers, query parameters and uri.
• Add methods for encoding headers, query parameters and uri sections and use these to construct the AWSHTTPRequest.
• Use the above to replace all usages of Mirror
• Add AWSHTTPBody, AWSEventStream encode(to:) that uses the above
• Conform ResponseDecodingContainer function names to the same as RequestEncodingContainer
• Allow XMLEncoder to return nil (if you encode a type whose encode function only has a singleValueContainer used to encode a nil value then there is no XML to serialise).

[codecov]

Codecov Report

Merging #576 (bd76668) into 7.x.x (076d383) will increase coverage by 2.11%.
The diff coverage is 76.85%.

❗ Current head bd76668 differs from pull request most recent head f2e77d9. Consider uploading reports for the commit f2e77d9 to get more accurate results

@@            Coverage Diff             @@
##            7.x.x     #576      +/-   ##
==========================================
+ Coverage   79.11%   81.22%   +2.11%     
==========================================
  Files          78       77       -1     
  Lines        6512     6457      -55     
==========================================
+ Hits         5152     5245      +93     
+ Misses       1360     1212     -148     

Files Changed	Coverage Δ
Sources/SotoCore/Concurrency/EventStream.swift	83.92% <0.00%> (-11.99%)	⬇️
Sources/SotoCore/AWSShapes/EventPayload.swift	40.00% <40.00%> (ø)
Sources/SotoCore/Encoder/RequestContainer.swift	72.72% <72.72%> (ø)
Sources/SotoCore/Encoder/ResponseContainer.swift	75.00% <87.50%> (+39.19%)	⬆️
Sources/SotoXML/XMLEncoder.swift	46.17% <88.46%> (+0.78%)	⬆️
Sources/SotoCore/HTTP/AWSHTTPRequest.swift	90.04% <94.87%> (-4.61%)	⬇️
Sources/SotoCore/Doc/AWSShape.swift	62.50% <100.00%> (+11.77%)	⬆️
Sources/SotoCore/Encoder/QueryEncoder.swift	66.89% <100.00%> (ø)
Sources/SotoCore/HTTP/AWSHTTPBody.swift	60.65% <100.00%> (+10.65%)	⬆️
Sources/SotoTestUtils/TestServer.swift	94.76% <100.00%> (ø)

... and 7 files with indirect coverage changes

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