Support linking crypto library for unit test purposes

[achamayou]

There is currently only a single way to consume libccf, to build full applications, but some of these applications, such as https://github.com/microsoft/scitt-ccf-ledger/, want to unit test functionality they are building that takes dependencies on individual CCF libraries, such as crypto.

In the specific case of crypto, there's an init function (openssl_sha256_init), which exists as a mitigation for OpenSSL 3.x digest context creation costs, which is not exposed in any public headers. Users have to resort to unfortunate workarounds.

We should:

• Confirm the separate init() is still the way to go. This was done in the Open Enclave/3.1 days, we've moved on to 3.3/SymCrypt on Azure Linux, perhaps this is no longer necessary.
• Have a clear, defined story for users who want to use ccfcrypto in their unit tests. That means a bit of doc with a list of headers, and updating at least some of the crypto unit tests to only include public headers/limited linking of ccfcrypto for demo purposes.