Add policy set for GitHub provider

[HorizonNet]

Add GitHub as a new provider with the existing policy copied over from
the gcp policies.

Resolves #325

[HorizonNet]

Tested locally via bin/terrascan scan -t github -p pkg/policies/opa/rego/github -f repositories.tf. Violations are shown as expected. Missing point is that violations from the original gcp policy is also reported when running bin/terrascan scan -t github -p pkg/policies/ -f repositories.tf.

[codecov]

Codecov Report

Merging #334 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #334      +/-   ##
==========================================
+ Coverage   61.42%   61.45%   +0.03%     
==========================================
  Files          56       57       +1     
  Lines        1068     1069       +1     
==========================================
+ Hits          656      657       +1     
  Misses        346      346              
  Partials       66       66              

Impacted Files	Coverage Δ
pkg/cli/scan.go	64.28% <ø> (ø)
pkg/policy/github.go	100.00% <100.00%> (ø)

[HorizonNet]

@williepaul Thanks for the feedback. Addressed your points. Let me make this one ready for review as you already gave feedback.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
0 Code Smells

No Coverage information
0.0% Duplication

[williepaul]

@HorizonNet all looks good, I tested the changes out locally and they worked fine. We really appreciate the contribution!