Fix optional plugin download logic

[tieneupin]

The get_plugin_wheel API endpoint previously did not generate the file with the correct name. This was due to a missing f-string in the "Content-Disposition" value in the header.

Additionally, a new API endpoint was added that returns a HTML page showing the plugins available for a given instrument.

Tests have also been added for these two functions as part of ongoing efforts to improve test coverage for this code base.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.05%. Comparing base (e74297e) to head (7e87a6c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #893      +/-   ##
==========================================
+ Coverage   54.99%   55.05%   +0.06%     
==========================================
  Files         103      103              
  Lines       11523    11528       +5     
  Branches     1531     1531              
==========================================
+ Hits         6337     6347      +10     
+ Misses       4843     4838       -5     
  Partials      343      343              

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