feat!: Share `Extension`s under `Arc`s #2485

Job	Run time
Check for changes	5s
check	1m 18s
Build benchmarks 🏋️	5m 50s
tests (Rust stable, all features)	2m 16s
tests (Rust stable, no features)	1m 49s
tests (Rust nightly, coverage)	3m 11s
tests (Rust 1.75)	2m 45s
tests (Rust beta)	3m 3s
rs-semver-checks / Rust semver-checks 🦀	3m 32s
Check standard extensions	6s
Required checks 🦀	1s
	23m 56s

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