Add draft PRs support

[kusaeva]

Hi!
Just added a boolean "draft" field indicating whether the pull request is a draft.

[codecov]

Codecov Report

Merging #577 (b167830) into master (354065b) will decrease coverage by 16.11%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master     #577       +/-   ##
===========================================
- Coverage   90.29%   74.17%   -16.12%     
===========================================
  Files          25       25               
  Lines         608      608               
  Branches        3        3               
===========================================
- Hits          549      451       -98     
- Misses         59      157       +98     

Impacted Files	Coverage Δ
...s/src/main/scala/github4s/domain/PullRequest.scala	0.00% <ø> (-100.00%)	⬇️
...ithub4s/interpreters/PullRequestsInterpreter.scala	100.00% <100.00%> (ø)
github4s/src/main/scala/github4s/Github.scala	0.00% <0.00%> (-100.00%)	⬇️
...ithub4s/src/main/scala/github4s/GithubConfig.scala	0.00% <0.00%> (-100.00%)	⬇️
...4s/src/main/scala/github4s/http/Http4sSyntax.scala	0.00% <0.00%> (-100.00%)	⬇️
.../src/main/scala/github4s/http/RequestBuilder.scala	0.00% <0.00%> (-100.00%)	⬇️
...s/src/main/scala/github4s/modules/GithubAPIs.scala	0.00% <0.00%> (-92.31%)	⬇️
...ub4s/src/main/scala/github4s/http/HttpClient.scala	9.75% <0.00%> (-85.37%)	⬇️
...cala/github4s/interpreters/StaticAccessToken.scala	0.00% <0.00%> (-50.00%)	⬇️
... and 3 more

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[BenFradet]

Thank you 👍