Return a cert chain we can use, ordered from leaf to root, or else an empty vector.
It is assumed that the caller can get the private key of the leaf with privateKeyFor
specifies the key types desired ("RSA", "DSA", "ECDSA", etc), or empty if there is no preference by the caller.
specifies the type of operation occuring
specifies a context relative to type.
See Implementation
Return a cert chain we can use, ordered from leaf to root, or else an empty vector.
It is assumed that the caller can get the private key of the leaf with privateKeyFor