odsign: Refactor into binary + library

Create libsigningutils library which contains functionality for
signing, signature verification, and fsverity. The goal is to allow
init to use some of this functionality during early boot.

Add a unit test to verify extractPublicKeyFromX509() and
verifySignature().

Bug: 199914227
Test: atest
Change-Id: Iea46539302df240cd2c92b1c4f313da773d8d180
12 files changed