Raise when unsupported db vendor, and add option to specify SQL analyser

[David-Wobrock]

Fixes #138
Fixes #169

[codecov-commenter]

Codecov Report

Merging #195 (ad189b0) into main (71ddadd) will decrease coverage by 0.18%.
The diff coverage is 73.68%.

@@            Coverage Diff             @@
##             main     #195      +/-   ##
==========================================
- Coverage   93.68%   93.50%   -0.19%     
==========================================
  Files          79       79              
  Lines        1758     1770      +12     
==========================================
+ Hits         1647     1655       +8     
- Misses        111      115       +4     

Impacted Files	Coverage Δ
django_migration_linter/sql_analyser/analyser.py	80.76% <61.53%> (-13.35%)	⬇️
...ation_linter/management/commands/lintmigrations.py	89.02% <100.00%> (+0.27%)	⬆️
django_migration_linter/migration_linter.py	84.38% <100.00%> (+0.04%)	⬆️
django_migration_linter/sql_analyser/__init__.py	100.00% <100.00%> (ø)
tests/unit/test_sql_analyser.py	100.00% <100.00%> (ø)

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