Tighten validation: only allow StaticPropSource in XB fields + PageTemplate, DynamicPropSource in ContentTypeTemplate

I expect this to fail like so:

1) Drupal\Tests\experience_builder\Kernel\Plugin\Field\FieldType\ComponentTreeItemTest::testInvalidField with data set "valid values using dynamic props" (['{"a548b48d-58a8-4077-aa04-da9...ts"}}}', '{"dynamic-static-card2df":{"h...ue"}}}'], [])
Failed asserting that two arrays are identical.
--- Expected
+++ Actual
@@ @@
-Array &0 []
+Array &0 [
+    'field_xb_test.0' => 'The 'dynamic' prop source type must be absent.',
+]

This has zero impact on the UI today, only on tests that need to be refactored a bit. @tedbow is best suited for that, because he worked on related validation constraints a few months ago and introduced ComponentTreeTestTrait which will need to have its uses of DynamicPropSources split out because many of those won’t be allowed anymore — as the cited test output failure shows.

Added a comment in #3455629-33: [later phase] [META] 7. Content type templates — aka "default layouts" — affects the tree+props data model → because that issue will need to add back some of what is being remove here related to translation.

Clearly I underestimated the complexity of landing this, because I'd completely forgotten about @tedbow's excellent TranslationTest that verifies the correct translation of the host entity's base/bundle field is being used! 🙈

Thanks, @tedbow, for so gracefully pushing this across the finish line 🙏

As far as I'm concerned, this is RTBC. This still needs @tedbow's approval on the MR, so @tedbow: this is all yours to merge 😊

I think the MR needs @f.mazeikis' approval because of the changes to `/config/schema/`

Looks good, approved

🚀 This implemented point 2 of the proposed resolution. Point 1 already happened previously. Points 3 and 4 have their own respective issues 👍