Lift http handler type discovery up a layer #5709

Job	Run time
Build Spin Ubuntu	18m 38s
all-integration-tests	28m 35s
Build Spin (windows-latest)	26m 59s
Build Spin static (aarch64, aarch64-unknown-linux-musl)	14m 32s
Build Spin (macos-latest)	16m 58s
Build Spin static (amd64, x86_64-unknown-linux-musl)	13m 51s
Lint Rust	6m 19s
Run release build steps (ubuntu-20.04, amd64, --features openssl/vendored, target/release)	13m 18s
Run release build steps (ubuntu-20.04, aarch64, --features openssl/vendored --target aarch64-unkn...	14m 46s
Test Spin (ubuntu-22.04-4core-spin)	10m 45s
Run release build steps (macos-latest, amd64, target/release)	17m 36s
Test Spin (macos-latest)	35m 50s
Run release build steps (macos-latest, aarch64, --target aarch64-apple-darwin, aarch64-apple-darw...	21m 47s
Run release build steps (windows-latest, amd64, .exe, target/release)	25m 54s
	4h 25m 48s

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