Authenticate file w/ fs-verity digest in hex

An input directory is configured by a build manifest file. With this
change, authfs starts to use the fs-verity digest in the manifest to
authenticate the corresponding file.

Although we're not using it, this change also makes it possible to
specify a known fs-verity digest to a remote file from the command line
(and deprecate the fake support of signature verification by the given
certificate).

Since we no longer derive fs-verity digest from the first chunk of a
Merkle tree, some relevant functions are deleted.

Bug: 206869687
Test: atest AuthFsHostTest ComposHostTestCases
Test: atest authfs_device_test_src_lib
Change-Id: Ibb5c246fb0d29aeafde187555f8d72c0282a65c7
diff --git a/authfs/src/fsverity/common.rs b/authfs/src/fsverity/common.rs
index 83e8ae4..eba379d 100644
--- a/authfs/src/fsverity/common.rs
+++ b/authfs/src/fsverity/common.rs
@@ -24,8 +24,8 @@
 
 #[derive(Error, Debug)]
 pub enum FsverityError {
-    #[error("Cannot verify a signature")]
-    BadSignature,
+    #[error("Invalid digest")]
+    InvalidDigest,
     #[error("Insufficient data, only got {0}")]
     InsufficientData(usize),
     #[error("Cannot verify a block")]