set up sql functions for computing performance matrix

[elfkuzco]

Rationale

This PR creates functions required to compute the performance of mirrors against country and region in the database. Through these functions, a table can be generated on Metabase to generate the rows. The functions are written in SQL as there is no native way of interacting with Python through Metabase. This lays the foundation for #40

NOTE
These functions assume that the mirror brain attributes as described in kiwix/container-images#263 have been set on the database.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.66%. Comparing base (b522855) to head (0c493b2).
Report is 8 commits behind head on main.

Additional details and impacted files

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##             main      #48   +/-   ##
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  Coverage   69.66%   69.66%           
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  Files          30       30           
  Lines         946      946           
  Branches       93       93           
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  Hits          659      659           
  Misses        267      267           
  Partials       20       20           

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[rgaudin]

Wow impressive work!

It all makes sense and looks good to me.