refactor: add `iroh_net::ConnManager` and use in `iroh_gossip::net` #8396

Job	Run time
Build wasm32	28s
check_semver	4m 22s
Checking fmt	6s
Checking docs	1m 52s
cargo deny	32s
clippy_check	1m 44s
Cross compile (i686-unknown-linux-gnu)	2m 39s
Run network simulations/benchmarks	7m 39s
Minimal Supported Rust Version	2m 0s
Cross compile (armv7-linux-androideabi)	1m 15s
Cross compile (aarch64-linux-android)	59s
CI Test Suite / Tests (ubuntu-latest, stable, all)	4m 20s
CI Test Suite / Tests (windows-latest, stable, all, x86_64-pc-windows-msvc)	4m 1s
CI Test Suite / Tests (ubuntu-latest, stable, none)	3m 1s
CI Test Suite / Tests (windows-latest, stable, none, x86_64-pc-windows-msvc)	3m 42s
CI Test Suite / Tests (ubuntu-latest, stable, default)	2m 58s
CI Test Suite / Tests (windows-latest, stable, default, x86_64-pc-windows-msvc)	3m 48s
CI Test Suite / Tests (macOS-arm-latest, stable, all)	3m 51s
CI Test Suite / Tests (macOS-arm-latest, stable, none)	3m 4s
CI Test Suite / Tests (macOS-arm-latest, stable, default)	3m 38s
	55m 59s

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