Add support for decoding event streams

[adam-fowler]

See S3.SelectObjectContent, Lambda.InvokeWithResponseStream etc. You can see documentation on decoding these here https://docs.aws.amazon.com/AmazonS3/latest/API/RESTSelectObjectAppendix.html.

• Added an AWSEventStream type that reads from a base ByteBuffer async sequence.
• These buffers are passed to an EventStreamDecoder which decodes the headers and then passes payload onto relevant decoder.

Related PRs: https://github.com/soto-project/soto-codegenerator/pull/74, https://github.com/soto-project/soto/pull/686

[codecov[bot]]

Codecov Report

Merging #566 (bbf4079) into 7.x.x (15744ff) will decrease coverage by 0.16%. The diff coverage is 81.31%.

@@            Coverage Diff             @@
##            7.x.x     #566      +/-   ##
==========================================
- Coverage   78.87%   78.72%   -0.16%     
==========================================
  Files          71       73       +2     
  Lines        6292     6487     +195     
==========================================
+ Hits         4963     5107     +144     
- Misses       1329     1380      +51     

Impacted Files	Coverage Δ
Sources/SotoCore/Doc/AWSShape.swift	50.72% <ø> (ø)
Sources/SotoCore/Encoder/ResponseContainer.swift	35.80% <0.00%> (-1.38%)	:arrow_down:
Sources/SotoCore/Encoder/EventStreamDecoder.swift	77.85% <77.85%> (ø)
Sources/SotoXML/Expat.swift	94.87% <80.00%> (+0.13%)	:arrow_up:
Sources/SotoCore/Concurrency/EventStream.swift	95.91% <95.91%> (ø)
Sources/SotoTestUtils/TestUtils.swift	84.61% <100.00%> (ø)

... and 6 files with indirect coverage changes

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