Add support for SSL_group_to_name and SSL_get_negotiated_group

[romen]

This PR adds support for OpenSSL 3.0 functions to retrieve information about the effective group negotiated in the TLS handshake from an SSL* object.

Marked as draft as I need more guidance from maintainers:

• cargo test fails with

  error: returning ‘const char * (*)(SSL *, int)’ {aka ‘const char * (*)(struct ssl_st *, int)’} from a function with incompatible 
         return type ‘const char * (*)(const SSL *, int)’ {aka ‘const char * (*)(const struct ssl_st *, int)’}
         [-Werror=incompatible-pointer-types]
  

  so I must be doing something wrong with the declaration, but I am struggling to see it

• in openssl/src/ssl/mod.rs:3523 I currently have an .expect() which is probably undesirable, but I'd like guidance on the proper way to handle that error to be consistent with the rest of the crate.

[sfackler]

I don't think we shouldn't be exposing this as a raw c_int. Since their doing the weird unknown thing, I think we'd need to make a NegotiatedGroup type that provides a pub fn nid(self) -> Option<Nid> and pub fn unknown_group_id(self) -> Option<i32>.

[romen]

I can do this change.

Then again in SSL::group_to_name which calls SSL_group_to_name(), we need the raw unaltered c_int.

Should the NegotiatedGroup type implement From/To trait to convert back to the raw c_int?
Suggestions on how to implement that?

[sfackler]

Yep, these wrapper types have an as_raw method to get the c_int out: https://docs.rs/openssl/latest/openssl/nid/struct.Nid.html#method.as_raw

[romen]

I tried addressing this with 893ce6a