Make TPM2 support self-contained
The TPM2 private key support in GnuTLS relies on other crypto libraries (OpenSSL or mbedTLS, depending on how tpm2-tss is compiled) for cryptographic primitives used by the ESYS API. Since version 4.0.0, tpm2-tss started providing an ability to switch crypto backend at run time, through Esys_SetCryptoCallbacks
. It would be nice if we could make use of it and set GnuTLS based callbacks.