[SCHEMA] Add a new file_relative format

[tsalo]

Closes None, but is relevant for #1023.

Changes proposed:

• Add a file_relative format to schema/objects/formats.yaml.
• Add tests for the new format.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.50%. Comparing base (7ec7450) to head (7cc9abf).
Report is 1435 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1046      +/-   ##
==========================================
+ Coverage   70.53%   71.50%   +0.96%     
==========================================
  Files           9        9              
  Lines         930      930              
==========================================
+ Hits          656      665       +9     
+ Misses        274      265       -9     

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[sappelhoff]

Thanks @tsalo