Libtrust key function

This PR adds a function for loading or creating the libtrust key.  This key function will be needed for changes related to identity-based TLS, provenance, and clustering.  The libtrust update contains changes related to using a standard method for generating the fingerprint.  It is important that any libtrust fingerprints use a standard method for generation to ensure key IDs may be matched across versions.

Example server usage to print the key ID
~~~~
trustKey, _ := api.LoadOrCreateTrustKey(job.Getenv("TrustKey"))
fmt.Println(trustKey.KeyID())
~~~~

Related to #8265