[SECENG-679] added eval policy subcommand to get raw opa evalution

[sagar-connect]

Checklist

=========

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have checked for similar issues and haven't found anything relevant.
• This is not a security issue (which should be reported here: https://circleci.com/security/)
• I have read Contribution Guidelines.

Changes

=======

• Added eval subcommand under policy command
• added unit-tests for eval subcommand
• moved owner-id flag declaration from policy root to individual subcommands

Rationale

=========

The rationale behind adding subcommand is allowing the advanced users of our circleci-policy-agent to obtain raw opa evaluation response and evaluate whether our helper functions are working as intended.

Considerations

==============

The owner-id flag isn't declared at root policy command level, because for eval subcommand it is not needed. and decide` subcommand it is optional.

[codecov]

Codecov Report

Merging #761 (b03b8d7) into master (f06195a) will increase coverage by 0.34%.
The diff coverage is 66.29%.

@@            Coverage Diff             @@
##           master     #761      +/-   ##
==========================================
+ Coverage   32.64%   32.99%   +0.34%     
==========================================
  Files          46       46              
  Lines        5413     5477      +64     
==========================================
+ Hits         1767     1807      +40     
- Misses       3390     3402      +12     
- Partials      256      268      +12     

Impacted Files	Coverage Δ
cmd/policy/policy.go	77.38% <66.29%> (-4.35%)	⬇️

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[davidmdm]

There's a lot of common code in getLocalDecision and getLocalEvaluation, (especially around loading policy files from the FS). I am not generally someone who recommends anything DRY. but this may be a good candidate.

[davidmdm]

Secondly I believe eval should take a positional argument (although a flag would be acceptable) to let them specify the query.

Sometimes all I want to know is if my helper rule has the output I expect, and as a customer I don't necessarily want to query the whole data document but only my helper.

circle policy eval --policy ./policy.rego --input ./config.yml --query data.org.my_helper_rule