Extension content verification: new class for parsing data from store

extensions/test/data/content_verifier/README

+
+The public/private key pairs were generated with the following commands:
+
+openssl genrsa -out private_key.pem 2048
+openssl rsa -in private_key.pem -pubout -out public_key.pem
+
+
+The signature was generated by:
+
+1) Take the contents of payload.json and base64url encode them:
+cat payload.json | tr -d \\n | base64 -w0 | tr _ / | tr \- + | tr -d '=' > payload_encoded.txt
+
+2) Put the contents of payload_encoded.txt into the "payload" field of
+verified_contents.json.
+
+3) Copy the contents of the "protected" field from verified_contents.json into
+protected.txt.
+
+4) Concatenate the "protected" and "payload" fields with a '.' separator.
+
+echo -n '.' | cat protected.txt - payload_encoded.txt > signature_input.txt
+
+5) Sign it
+
+tr -d \\n < signature_input.txt | openssl dgst -sha256 -sign private_key.pem -binary | base64 -w0  | tr _ / | tr \- + | tr -d '=' > signature.txt
+
+6) Put the contents of signature.txt into the "signature" field in
+verified_contents.json.