refactor: add `iroh_net::ConnManager` and use in `iroh_gossip::net` #8404

Job	Run time
Build wasm32	30s
Checking docs	1m 10s
Checking fmt	9s
check_semver	4m 53s
Cross compile (i686-unknown-linux-gnu)	2m 35s
Cross compile (armv7-linux-androideabi)	1m 1s
cargo deny	30s
clippy_check	2m 16s
Cross compile (aarch64-linux-android)	1m 2s
Minimal Supported Rust Version	1m 16s
Run network simulations/benchmarks	7m 47s
CI Test Suite / Tests (ubuntu-latest, stable, all)	4m 17s
CI Test Suite / Tests (ubuntu-latest, stable, none)	2m 47s
CI Test Suite / Tests (windows-latest, stable, all, x86_64-pc-windows-msvc)	3m 52s
CI Test Suite / Tests (ubuntu-latest, stable, default)	2m 58s
CI Test Suite / Tests (windows-latest, stable, none, x86_64-pc-windows-msvc)	3m 58s
CI Test Suite / Tests (windows-latest, stable, default, x86_64-pc-windows-msvc)	3m 40s
CI Test Suite / Tests (macOS-arm-latest, stable, all)	3m 51s
CI Test Suite / Tests (macOS-arm-latest, stable, none)	3m 1s
CI Test Suite / Tests (macOS-arm-latest, stable, default)	3m 40s
	55m 13s

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