Go Lint fixes and GitHub actions added for Test, Coverage and Golint

[Chetan177]

Changes:

• Go Lint fix for the following:

  constants/error_code.go:4:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/error_code.go:4:2: exported const BAD_REQUEST_ERROR should have comment (or a comment on this block) or be unexported
  constants/error_code.go:5:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/error_code.go:6:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/http_status_code.go:7:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/http_status_code.go:9:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/url.go:5:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/url.go:7:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/url.go:9:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/url.go:11:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/url.go:13:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/url.go:15:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/url.go:17:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/url.go:19:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/url.go:21:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/url.go:23:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/url.go:25:2: don't use ALL_CAPS in Go names; use CamelCase
  constants/url.go:27:2: don't use ALL_CAPS in Go names; use CamelCase

• Added Test, Coverage, Lint check using Git Action instead of TravisCI https://github.com/razorpay/razorpay-go/issues/11

[Chetan177]

Please let me know if code coverage through codecov.io is required.

It will show code coverage on codecov.io like this:

And will also add a comment on the related PR like this: