Add TKM private key implementation

Add TKM private key implementation

The key currently imitates the private key of alice@strongswan.org by
returning it's fingerprint in the get_fingerprint function.

This associates the private key with alice's X.509 cert and charon will
use it to create a signature over the local AUTH octets of the test
connection.

The private key serves as a proxy to the TKM ike_isa_sign operation and
extracts the required information from the auth octets chunk passed on
by the keymat.