Add support for (re)storing state for Digest operations

[simo5]

NSS expects to be able to store the state of hashing operations and then restore it later.
This is used to deal with the TLS transaction hash as the store/restore operation operates effectively like a context duplication in OpenSSL.

Ideally OpenSSL would provide a way to export the dgest state so we could actually save it, that request is tracked openssl/openssl#14222 but currently unimplemented.

So we'll need to address is in a different way unless we find the bandwidth to contribute a way to serialize data in OpenSSL.

One way is to re-implement digest functions so we can directly access and serialize the state ourselves.

A different way to approach this could be to hand on a session a list of "duplicated" contexts with a unique id, and when an application "restores" a context, we can search for the unique id, and duplicate again the OpenSSL context in the new session operation.

The reason why we'd hang these context on a session is that we need a way to free these OpenSSL contexts eventually.

This would allow simulating store/restore without having to leak memory completely, however if would depend on the original session to stay open until the restore operation is called.