Add basic config validation constraints

Hi I just tried to create a basic choice contraint following these docs https://www.drupal.org/node/3376195
https://symfony.com/doc/current/reference/constraints/Choice.html#basic-usage

Thank you for the effort, but that MR is not really helpful.

- The validation constraint definition needs to be conditionally backported, as the CKEditor5 module previously did: https://git.drupalcode.org/project/drupal/-/merge_requests/4352/diffs#18...
- The csp_directive_source_list.base schema can be a static list of options.
- csp_directive_source_list.flags could be limited to a static list of options. I don't know that the callback is given enough context to limit flags that aren't valid for each directive (e.g. unsafe-inline doesn't make sense if not on default-src, script-src, or style-src)
- csp_policy.reporting.plugin could possibly be limited by a callback to the available reporting plugins

gapple → closed merge request !15

I think easier to require ^10.2 for 2.0.0, and skip backporting support for <=10.1 in the 8.x-1.x branch.

I think having this as part of 2.1.0, after core support for 8.1 is dropped, since this isn't necessary to introduce in 2.0.0.

This can be added to 2.2.0 or later

There's some additional validation constraints that are useful:
- The reporting plugin id →
- Required / optional mapping keys

Some possible future enhancements:
- Only permit the 'flags' property on relevant directives
- Validate the 'flags' values permitted for the respective directive
- Validate directives sequence keys

Automatically closed - issue fixed for 2 weeks with no activity.