Add support for Image4 Certificates (IM4C)

[justtryingthingsout]

These appear in particular cases, such as AuthDebug responses or Yonkers patch IM4Ms.
The format as listed by dumpasn1 is as follows:

 487  525:   SEQUENCE {
 491  521:     SEQUENCE {
 495    4:       IA5String 'IM4C'
 501    1:       INTEGER 0
 504  436:       SET {
 508  354:         [PRIVATE 1129469008] {
 517  350:           SEQUENCE {
 521    4:             IA5String 'CRTP'
 527  340:             SET {
...

IM4C appears to mean Image4 Certificates, and CRTP as Certificate Properties, and they appear to simply be the Apple constraint extensions usually embedded in certificates, except now embedded in this IM4C wrapper.