Change feature detection logic for database field

[sevdog]

As of now HexIntegerField relies on connection.settings_dict["ENGINE"] to determinate if the database supports specific feature.

Since a custom engine may be defined by any user this has some limitation because engines are hard-coded.

The preferred approach would be to use connection.feature_class.<feature>, but ATM there is no flag in these classes which may help here (ie: supports_unsigned_integers).

My solution here is to rely on connection.vendor because it should be more robust and could handle subclassing of any existent backend, if that backend has some kind of differences it could just override vendor attribute (ie: postgresql -> custom-postgresql).

Also this saves us some configurations since django.db.backends.postgresql, django.db.backends.postgresql_psycopg2 and django.contrib.gis.db.backends.postgis have all vendor = "posrgresql", and we also get spatiallite (which is based on sqlite and has the same datatypes).

[codecov[bot]]

Codecov Report

Merging #612 (217d5b0) into master (13a2c6f) will decrease coverage by 0.02%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #612      +/-   ##
==========================================
- Coverage   68.30%   68.27%   -0.03%     
==========================================
  Files          24       24              
  Lines        1101     1100       -1     
  Branches      173      173              
==========================================
- Hits          752      751       -1     
  Misses        312      312              
  Partials       37       37              

Impacted Files	Coverage Δ
push_notifications/fields.py	60.93% <50.00%> (-0.61%)	:arrow_down:

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