Openpay integration

[SagarKarwande]

Why?

• Introduced new gateway Openpay.
• Added Unit tests.
• Added Integration tests.

  This change addresses the need by:

• Adding new Gateway to Gringotts with unit and Integration test cases.

[codecov-io]

Codecov Report

Merging #189 into dev will increase coverage by 1.73%. The diff coverage is 97.22%.

@@            Coverage Diff             @@
##              dev     #189      +/-   ##
==========================================
+ Coverage   77.39%   79.12%   +1.73%     
==========================================
  Files          14       15       +1     
  Lines         376      412      +36     
==========================================
+ Hits          291      326      +35     
- Misses         85       86       +1     

Impacted Files	Coverage Δ
lib/gringotts/gateways/openpay.ex	97.22% <97.22%> (ø)

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[mandarvaze]

@gopalshimpi PR description added. I think you need to approve, not just LGTM :)