feat: allow OpenTelemetry context access from SpanRef #281

Job	Run time
cargo check	41s
warnings	45s
cargo fmt	9s
cargo check (+MSRV -Zminimal-versions) (stable)	30s
cargo test (nightly on ubuntu-latest)	1m 1s
cargo test (beta on ubuntu-latest)	59s
cargo test (stable on macos-latest)	1m 31s
cargo check (feature combinations)	18s
cargo test (stable on ubuntu-latest)	1m 0s
cargo check (+MSRV -Zminimal-versions) (1.75.0)	33s
build tests (wasm32-wasip1)	57s
build tests (wasm32-unknown-unknown)	59s
cargo test (stable on windows-latest)	3m 20s
all systems go!	2s
	12m 45s

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