* consolidate (un)marshalling of key PEMs
This commit consolidates various functions we have for marshalling
private and public keys to and from the PEM format, mostly by replacing
functions from lib/utils and lib/tlsca with equivalents in api/utils/keys.
The new functions also support ECDSA and Ed25519 keys, which is
necessary for the implementation of RFD136.
* speed up TestMTLSClientCAs
* fix new use in local proxy middleware