🩺 Add Health Check and Sessions Endpoints

[aacic]

This PR introduces the healthcheck endpoint and the sessions endpoint.

The purpose of the healthcheck endpoint is to tell if the TileServer is running.

The purpose of the sessions endpoint is to return a mapping of all active sessions and their loaded Whole Slide Images (WSIs).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.70%. Comparing base (bae6c2e) to head (1fbf80e).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #925   +/-   ##
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  Coverage    99.70%   99.70%           
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  Files           71       71           
  Lines         8848     8859   +11     
  Branches      1154     1156    +2     
========================================
+ Hits          8822     8833   +11     
  Misses          23       23           
  Partials         3        3           

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