Feature/string db credentials

[acelaya]

Closes #2058

Make sure database user and password are casted to string when read as env vars.

This is a workaround for a problem caused by how env vars are read and type-casted in Shlink. Right now, they are converted to a specific type based in their value, while they should probably be type-casted based on the context they are going to be used on.

However, this requires more changes and considerations, and it could end up introducing other problems.

This just fixes the specific problems reported in #2058, and delays any further changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.34%. Comparing base (a3d5060) to head (293725f).
Report is 3 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #2062   +/-   ##
==========================================
  Coverage      97.34%   97.34%           
  Complexity      1363     1363           
==========================================
  Files            251      251           
  Lines           4826     4826           
==========================================
  Hits            4698     4698           
  Misses           128      128           

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