X509Certificate: refactor initialization, make members const

Move the parsed fields into a subclass and parse before trying to create
the X509Certificate, thus allowing to make all the X509Certificate
members const.

Moving all the parsed fields into a single subclass also makes
it easier implementing the CloneWithDifferentIntermediates method in the
followup CL.

Change-Id: Iae86b8bd8eed53c6cf3acb1c9c07ad09e86d4f66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4549194


Reviewed-by: David Benjamin <davidben@chromium.org>
Commit-Queue: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1146753}