Add database-backend-specific checks

[Shai Berger]

Different database backends have different features and behaviors. As a result, some definitions which may be ok on some backends, may warrant warnings or even errors on other backends.

As one example, a 65-chars-long field name is a problem on PostgreSQL, but not on any other core backend; as another, column named "timestamp" warrants a warning on Oracle (it might work or not...) but not anywhere else.

The place where such definitions are checked is the checks framework; and to account for the different behaviors, we need database-backend-specific checks.

This is a spin-off from #13711 -- which, in its last iteration, needed special care for the field-name-too-long check.

[Tim Graham]

I haven't looked into details, but I just came across django/db/backends/mysql/validation.py which implements MySQL specific field validation.