libfs_avb: adding two public APIs

The client can include <fs_avb/fs_avb_util.h> to use the two new
functions to load vbmeta for a FstabEntry and extract the hash tree
descriptor from the loaded vbmeta, respectively.

// Given a FstabEntry, loads and verifies the vbmeta.
std::unique_ptr<VBMetaData> LoadAndVerifyVbmeta(...);

// Gets the hashtree descriptor with avb_partition_name from the vbmeta.
std::unique_ptr<FsAvbHashtreeDescriptor> GetHashtreeDescriptor(...);

Bug: 65470881
Test: atest libfs_avb_test
Test: atest libfs_avb_internal_test
Test: atest libfs_avb_device_test
Change-Id: I7d6619eb8140c14734ffb8f8a1b22cddd2f562f0