Add basic functionality for running graphql requests

[dennisgranath]

• This adds a method for posting requests to the graphql endpoint.
• It also adds the nodeId field to the PullRequest model (this is needed to be able to reference PR:s in graphql mutations)

[codecov]

Codecov Report

Attention: Patch coverage is 31.03448% with 20 lines in your changes are missing coverage. Please review.

Project coverage is 76.15%. Comparing base (b8e8ae6) to head (16cb788).

Files	Patch %	Lines
...va/com/spotify/github/v3/clients/GitHubClient.java	31.03%	20 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #182      +/-   ##
============================================
- Coverage     77.28%   76.15%   -1.13%     
- Complexity      312      313       +1     
============================================
  Files            44       44              
  Lines          1039     1057      +18     
  Branches         44       44              
============================================
+ Hits            803      805       +2     
- Misses          211      227      +16     
  Partials         25       25              

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