Split pkinit_identity_initialize into two phases

Split pkinit_identity_initialize into two phases

Split part of pkinit_identity_initialize() into a second piece named
pkinit_identity_prompt().  Have each piece pass a new boolean flag to
crypto_load_certs() to indicate if it should defer prompting for a
password/PIN for client identities that require one.  If the flag isn't
set, then crypto_load_certs() should attempt to use a responder-supplied
value, or call the prompter if there isn't one.

ticket: 7680