Factor out shared http2 server transport code #2541

Job	Run time
Integration Tests on swiftlang/swift:nightly-jammy	4m 24s
Performance Tests on swiftlang/swift:nightly-jammy	3m 2s
Build and Test on swiftlang/swift:nightly-jammy	2m 4s
Integration Tests on swiftlang/swift:nightly-6.0-jammy	10m 4s
Performance Tests on swiftlang/swift:nightly-6.0-jammy	3m 25s
License Header and Formatting Checks	3m 4s
Integration Tests on swift:5.10.1-noble	5m 27s
Build and Test on swiftlang/swift:nightly-6.0-jammy	7m 27s
Integration Tests on swift:5.9-jammy	4m 9s
Build and Test on swift:5.10.1-noble	4m 8s
Integration Tests on swift:5.8-focal	3m 21s
Build and Test on swift:5.9-jammy	3m 17s
Performance Tests on swift:5.10.1-noble	33s
Build and Test on swift:5.8-focal	2m 56s
Performance Tests on swift:5.9-jammy	33s
Performance Tests on swift:5.8-focal	28s
	58m 22s

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