Add hash pointer to the ecmh finalize function in secp256k1

[elichai]

The common interface in secp256k1 is to pass a function pointer to the hash function so you can easily replace the hash function if needed.
that is missing in the ECMH interface

secp256k1/include/secp256k1_multiset.h

Line 124 in b8c5c3f

SECP256K1_API int secp256k1_multiset_finalize(

After the C API will be implemented for ECMH finalize, it can be used like the rest of the functions (schnorr/ecdsa)

In Schnorr we now are passing NULL which makes it use an internal C impl of sha256, but if we'll need to replace it - it is possible to pass a go function https://github.com/golang/go/wiki/cgo#function-pointer-callbacks

Probably the simplest way would be to implement a dummy hash function in go-secp that will just return the coordinates and then hash them via the supplied go callback outside of C.

something like: rust-bitcoin/rust-secp256k1#201