Add `internal` feature for `validate_num_of_signers` #1310

Job	Run time
actionlint	6s
build with MSRV (1.66.1)	20s
build with default features	28s
build with no_std (ristretto255)	26s
Clippy	32s
Check Rust doc	33s
build with no_std (ed25519)	21s
Rustfmt	18s
build with no_std (p256)	20s
Check if automatically generated code is up to date	30s
test on beta	2m 5s
build with no_std (secp256k1)	22s
build with no_std (rerandomized)	23s
	6m 44s

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