Add initial TKM Diffie-Hellman implementation

Add initial TKM Diffie-Hellman implementation

The tkm_diffie_hellman_t plugin acquires a DH context from the Trusted
Key Manager and uses it to get a DH public value and the calculated
shared secret. Proper context handling is still missing though, the
plugin currently uses context ID 1.

The get_shared_secret function will be removed as soon as the TKM
specific keymat is ready.