Add GPG signature verification on manifest load

- Implement gpgVerify function that creates a temporary keyring to verify
  detached signatures against embedded public keys
- Signature verification happens during deserialization after hash
  validation but before decompression
- Extract signatureString() as a method on manifest for generating the
  canonical signature string (MAGIC-UUID-MULTIHASH)
- Add --require-signature flag to check command to mandate signature from
  a specific GPG key ID
- Expose IsSigned() and Signer() methods on Checker for signature status

internal/cli/mfer.go

@@ -181,6 +181,12 @@ func (mfa *CLIApp) run(args []string) {
 						Name:  "no-extra-files",
 						Usage: "Fail if files exist in base directory that are not in manifest",
 					},
+					&cli.StringFlag{
+						Name:    "require-signature",
+						Aliases: []string{"S"},
+						Usage:   "Require manifest to be signed by the specified GPG key ID",
+						EnvVars: []string{"MFER_REQUIRE_SIGNATURE"},
+					},
 				),
 			},
 			{