Config overhaul: lowercase for `DatabaseDriver`

[josecelano]

Use lowercase for database driver values in the configuration:

[core.database]
driver = "sqlite3"
#driver = "MySQL"

Instead of:

[core.database]
driver = "Sqlite3"
#driver = "MySql"

We are normalizing all enum variants in the configuration to lowercase.

It also decouples the internal database driver enum from the enum used in the configuration.

[josecelano]

ACK ca348a839867ec56a66b0ce8abfd714d7633ede8

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 59.25926% with 11 lines in your changes missing coverage. Please review.

Project coverage is 77.31%. Comparing base (c747321) to head (ca348a8).

Files	Patch %	Lines
src/core/databases/error.rs	18.18%	9 Missing :warning:
src/core/databases/driver.rs	75.00%	1 Missing :warning:
src/core/mod.rs	75.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #953 +/- ## =========================================== - Coverage 77.31% 77.31% -0.01% =========================================== Files 183 183 Lines 9731 9735 +4 =========================================== + Hits 7524 7527 +3 - Misses 2207 2208 +1 ```

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