Introduce `EntityFieldBasedPropExpressionInterface::getFieldDefinition()`
> **Follow-up of** [!1112](https://git.drupalcode.org/project/canvas/-/merge_requests/1112) — see [note 1200323](https://git.drupalcode.org/project/canvas/-/merge_requests/1112#note_1200323).
### Overview
Logic added in !1112 resolves a field definition from an entity-field-based prop expression in a way that is essentially identical to `\Drupal\canvas\PropExpressions\StructuredData\FieldPropExpression::calculateDependencies()`. The same field-definition-resolution logic now exists in more than one place.
### Proposed resolution
Add `EntityFieldBasedPropExpressionInterface::getFieldDefinition()` and route the duplicated logic through it, so every entity-field-based prop expression resolves its field definition the same way.
### Remaining tasks
- Add `getFieldDefinition()` to `EntityFieldBasedPropExpressionInterface`.
- Implement it on the relevant prop expression classes.
- Replace the duplicated resolution logic (including in `FieldPropExpression::calculateDependencies()`) with calls to it.
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