releasetools: Add PayloadSigner class.

Create a wrapper class that handles the payload signing, which unifies
the paths with and without external signer. Also add tests for the newly
added class.

The test keys under testdata/ are created with the script in
development/tools/make_key. testdata/testkey_with_passwd.pk8 uses
password "foo".

Test: python -m unittest test_ota_from_target_files
Test: Get identical A/B OTA packages w/ and w/o the CL.
Change-Id: Ic770aec726498a3babb88ec509985e7f1210fb18

tools/releasetools/testdata/payload_signer.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+
+# The script will be called with 'payload_signer.sh <key> -in <input> -out <output>'.
+openssl pkeyutl -sign -keyform DER -inkey $1 -pkeyopt digest:sha256 -in $3 -out $5