refactor: extract `ProtocolHandler` impl into here #5

Job	Run time
cargo deny	29s
Android Build Only (aarch64-linux-android)	1m 6s
Android Build Only (armv7-linux-androideabi)	1m 7s
Checking docs	1m 6s
Checking fmt	43s
check_semver	15s
clippy_check	1m 33s
codespell	4s
Cross Test (i686-unknown-linux-gnu)	59s
Cross Build Only (x86_64-unknown-freebsd)	44s
Minimal Supported Rust Version	1m 4s
CI Test Suite / Tests (ubuntu-latest, stable, all)	1m 30s
CI Test Suite / Tests (windows-latest, stable, all, x86_64-pc-windows-msvc)	2m 1s
CI Test Suite / Tests (ubuntu-latest, stable, none)	27s
CI Test Suite / Tests (windows-latest, stable, none, x86_64-pc-windows-msvc)	1m 0s
CI Test Suite / Tests (ubuntu-latest, stable, default)	1m 26s
CI Test Suite / Tests (macOS-arm-latest, stable, all)	1m 16s
CI Test Suite / Tests (windows-latest, stable, default, x86_64-pc-windows-msvc)	1m 37s
CI Test Suite / Tests (macOS-arm-latest, stable, none)	31s
CI Test Suite / Tests (macOS-arm-latest, stable, default)	48s
	19m 46s

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