We have CBS_get_asn1 / CBS_get_asn1_element, but not the "any" variants
of them. Without this, a consumer walking a DER structure must manually
CBS_skip the header, which is a little annoying.

Change-Id: I7735c37eb9e5aaad2bde8407669bce5492e1ccf6
Reviewed-on: https://boringssl-review.googlesource.com/11404
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>