Add support to get Installation identified by its installation id

[miwurster]

Hi, this PR adds support to get the an installation's information using the installation id. Essentially, it enables to use the /app/installations/{installation_id} API endpoint.

[ebk45]

Hi @miwurster.

We apologise for taking so long to get eyes on this PR, we haven't been able to maintain this library to the standard we would have liked to but this will be changing in the new year. If you could please get this rebased and we'll be prioritising all open PRs ready for review.

Thanks!

Ellie

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (24b73da) 76.64% compared to head (331bf5c) 76.69%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #151      +/-   ##
============================================
+ Coverage     76.64%   76.69%   +0.04%     
- Complexity      289      290       +1     
============================================
  Files            42       42              
  Lines           989      991       +2     
  Branches         43       43              
============================================
+ Hits            758      760       +2     
  Misses          206      206              
  Partials         25       25              

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[miwurster]

Rebased and ready for review.

[felix-seifert]

Clear and concise contribution. 👍

At first sight, I just had to think about what happens if calling the URL results in not finding the installation or gives another bad response. I then saw that this is handled in our GitHubClient and does not have to be tested several times.