Move the SQLite and PostgreSQL code from the Views StringArgument to the sqlite and pgsql module

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Rerolled the patch.

Fixed failed commands on #11
Attached patch against Drupal 10.1.x

The Needs Review Queue Bot → tested this issue. It fails the Drupal core commit checks. Therefore, this issue status is now "Needs work".

Apart from a re-roll or rebase, this issue may need more work to address feedback in the issue or MR comments. To progress an issue, incorporate this feedback as part of the process of updating the issue. This helps other contributors to know what is outstanding.

Consult the Drupal Contributor Guide → to find step-by-step guides for working with issues.

I've adjusted the patch to fix the custom commands failing.

Moved the code I set in HandlerBase to ArgumentPluginBase as this is directly used by the StringArgument class. This way it doesn't get set for all classes extending the HandlerBase.

It is better to do it in the class it is actually used in, here's a updated patch.

All code changes look good to me.
The IS and the CR are in order.
For me it is RTBC.

This looks OK, but there is also Drupal\views\Plugin\views\query\SqliteDateSql - is there an issue to move this, and I wonder if it's worth having a single backend-overridable Views database service instead of multiple services?

Two nits:

1. +++ b/core/modules/pgsql/src/Plugin/views/argument/ArgumentStringService.php
   @@ -0,0 +1,31 @@
   + * Database neutral implementation of the service "views.argument_string".
   

   This class isn't database neutral.

2. +++ b/core/modules/sqlite/src/Plugin/views/argument/ArgumentStringService.php
   @@ -0,0 +1,32 @@
   + * Database neutral implementation of the service "views.argument_string".
   

   Neither is this one.