Add support to get a user installation for the authenticated app

[annelled]

What
Add support to get a user installation for the authenticated app.

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 76.26%. Comparing base (a8848f5) to head (2aa4e8c).

Files	Patch %	Lines
...va/com/spotify/github/v3/clients/GitHubClient.java	0.00%	1 Missing ⚠️
...java/com/spotify/github/v3/clients/UserClient.java	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #191      +/-   ##
============================================
+ Coverage     76.17%   76.26%   +0.08%     
- Complexity      315      317       +2     
============================================
  Files            44       44              
  Lines          1062     1066       +4     
  Branches         44       44              
============================================
+ Hits            809      813       +4     
  Misses          227      227              
  Partials         26       26              

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[dennisgranath]

LGTM. I see that coverage complains about tests. Should be relatively easy to add that similarly to the getInstallation endpoint.