support for signatures

At least [AWS.jl](https://github.com/JuliaCloud/AWS.jl/issues/696) and [JWTs.jl](https://github.com/JuliaWeb/JWTs.jl/issues/36) could potentially benefit from support for message signatures in this package. Between those two, there is reason to want support for at least HMAC, RSA, and EdDSA signatures.

It may be best to start with the [EVP_DigestSign](https://docs.openssl.org/3.2/man3/EVP_DigestSignInit) interface, which appears to support a wide range of algorithms.