refactor: add `iroh_net::ConnManager` and use in `iroh_gossip::net` #8393

Job	Run time
check_semver	16s
Checking fmt	9s
Checking docs	28s
Cross compile (i686-unknown-linux-gnu)	30s
Build wasm32	27s
Cross compile (armv7-linux-androideabi)	30s
Cross compile (aarch64-linux-android)	31s
cargo deny	14s
clippy_check	27s
Minimal Supported Rust Version	28s
Run network simulations/benchmarks	31s
CI Test Suite / Tests (ubuntu-latest, stable, all)	1s
CI Test Suite / Tests (ubuntu-latest, stable, none)	1s
CI Test Suite / Tests (ubuntu-latest, stable, default)	30s
CI Test Suite / Tests (windows-latest, stable, all, x86_64-pc-windows-msvc)	18s
CI Test Suite / Tests (macOS-arm-latest, stable, all)	30s
CI Test Suite / Tests (windows-latest, stable, none, x86_64-pc-windows-msvc)	1s
CI Test Suite / Tests (macOS-arm-latest, stable, none)	1s
CI Test Suite / Tests (windows-latest, stable, default, x86_64-pc-windows-msvc)	18s
CI Test Suite / Tests (macOS-arm-latest, stable, default)	30s
	6m 41s

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