feat: add LazyValue::as_cow_str #744

Job	Run time
fuzz	1m 49s
Rust nightly (macos-latest)	2m 22s
Rust stable (Windows)	4m 17s
Miri	1m 35s
Rust stable (ubuntu-24.04-arm)	2m 5s
Format check	26s
Sanitize address,leak feature non_trailing_zero	1m 48s
Sanitize address,leak feature utf8_lossy	1m 47s
Sanitize address,leak feature sort_keys	2m 9s
Rust nightly (ubuntu-24.04-arm)	2m 12s
Rust stable (ubuntu-latest)	1m 42s
Sanitize address,leak feature	1m 47s
test-stable-wasm	6m 46s
Rust stable (macos-latest)	2m 28s
Sanitize address,leak feature arbitrary_precision	1m 52s
Rust nightly (ubuntu-latest)	1m 44s
	36m 49s

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