nebraska: Rename some tags

Rename some tags to conform to what we generate in payload.json files
already. These changes include:
- AppData: version -> target_version
- AppData: src_version -> source_version
- AppData: metadata_sig -> metadata_signature
- AppData: hash_sha256 -> sha256_hex
- AppData: SHA256_HASH -> SHA256_HEX

TEST='./nebraska --target-payloads=./' and sent a request and got a correct reply

Change-Id: Id6039121144d3d0ae7887597f5b98e3d23da37d5
Commit-Ready: ChromeOS CL Exonerator Bot <>
Tested-by: Amin Hassani <>
Reviewed-by: Nicolas Norvez <>
6 files changed