Fix and test sr25519 signing in nostd #5480

Job	Run time
Cargo fmt	17s
Check unused dependencies	26s
Cargo clippy	3m 16s
Cargo check	8m 59s
Cargo clippy (WASM)	48s
Check documentation and run doc tests	4m 40s
Cargo check (WASM)	1m 20s
Test (no_std)	1m 52s
Test (Light Client)	7m 23s
Test (Native)	11m 3s
Test chainhead backend	11m 12s
Test (WASM)	11m 6s
	1h 2m 22s

Provide feedback