KBS: Be more tolerant to policy rego files

[davidhadas]

KBC fails to set policy in KBS and AS unless the rego file ends with an empty line.
Either make the code more tolerant or make it clear in docuemtation.

Example:

$ cat -n allow_all.rego
     1
     2	package policy
     3
     4	default allow = true

$ cat -n allow_all_modified.rego
     1
     2	package policy
     3
     4	default allow = true
     5

$ kbs-client --url "http://192.168.122.182:30713" config --auth-private-key ./privateKey set-resource-policy --policy-file allow_all.rego
Error: Request Failed, Response: "{\"type\":\"https://github.com/confidential-containers/kbs/errors/PolicyEndpoint\",\"detail\":\"Policy error: Set policy error Base64 decode OPA policy string failed: InvalidPadding\"}"

$ kbs-client --url "http://192.168.122.182:30713" config --auth-private-key ./privateKey set-resource-policy --policy-file allow_all_modified.rego
Set resource policy success
 policy: CnBhY2thZ2UgcG9saWN5CgpkZWZhdWx0IGFsbG93ID0gdHJ1ZQoK

same issue with set-attestation-policy

[Xynnn007]

cc @jialez0 this might be related to encoding things

[fitzthum]

Duplicate of #281

#357 might help with this (note that we add a test for a policy with extra lines) but this issue might happen before we actually get to the OPA module and that PR will only fix the resource policy not the attestation one.

Either way, maybe we should close this and discuss on the existing issue.