These changes add support for digital signatures for policy bundles which
can be used to verify their authenticity.
Bundle signature verification involves the following steps:
* Verify the JWT signature
* Verify the files in the JWT payload exist in the bundle
* Verify the file content of the files in bundle match with those in the payload
This commit adds a new `sign` command to generate a digital signature for policy bundles.
For more details, run "opa sign --help"
The signatures generated by the 'sign' command can be verified by the
'build' command. The 'build' command can also sign the bundle it generates.
The 'run' command can verify a signed bundle or skip verification altogether.
OPA 'sign', 'build' and 'run' can be used to
sign/verify bundles in bundle mode (--bundle) mode only. Verification
can be also be performed when bundle downloading is enabled.
Fixes: #1757
Signed-off-by: Ashutosh Narkar <anarkar4387@gmail.com>