feat: add diff method PullRequestClient

[mkarimi23]

Currently there is no support for getting the diff for a pullrequest. This is done by passing a certain header (application/vnd.github.diff) which is done in this PR

[codecov[bot]]

Codecov Report

Merging #149 (43c7137) into master (1ffd719) will increase coverage by 0.01%. The diff coverage is 77.77%.

@@             Coverage Diff              @@
##             master     #149      +/-   ##
============================================
+ Coverage     75.64%   75.66%   +0.01%     
- Complexity      279      281       +2     
============================================
  Files            42       42              
  Lines           969      978       +9     
  Branches         42       43       +1     
============================================
+ Hits            733      740       +7     
- Misses          211      212       +1     
- Partials         25       26       +1     

Files Changed	Coverage Δ
...m/spotify/github/v3/clients/PullRequestClient.java	47.76% <77.77%> (+4.65%)	:arrow_up: