Wrapper for OAuth URLs

[bhavyay]

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Test Case Document URL
https://docs.google.com/spreadsheets/d/1VlHhQWwJBN0y3WMumA5upH2h52WYi61SxoTKr4XyXy8/edit?usp=sharing

PR Description: Adding support for OAuth URLS(fetch authorize URL, access token, refresh token & revoke token APIs) in SDK. Also, adding support to access Razorpay Public APIs with access token Solution doc: https://docs.google.com/document/d/1mW1Ql08vndWIB_PjS-HWM_R2Nr63isW3jPsewYbTycE/edit

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.94656% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 96.83%. Comparing base (9423e2b) to head (bfb0bde).

Files	Patch %	Lines
lib/razorpay/payload_validator.rb	93.47%	3 Missing :warning:
lib/razorpay/oauth_token.rb	97.91%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #242 +/- ## ======================================== Coverage 96.83% 96.83% ======================================== Files 35 38 +3 Lines 631 758 +127 ======================================== + Hits 611 734 +123 - Misses 20 24 +4 ```

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