Deprecate drupal_schema_get_field_value()

Looks great to me!

Added credit for everyone who worked on this part of the patch from the parent issue.

Fixing the credits after reading the parent issue again, I forgot quite a few people. Sorry! :(

Committed ecf0410 and pushed to 8.8.x. Thanks!

Discussed this with @amateescu, @Berdir and @larowlan a bit. Whilst I think burying this in entity storage is in someways sub-optimal - this is one fo those hard choices and sat least we've marked it internal. Hopefully at some point we can remove this entirely and replace it with PHP strict typing and scalar type hints.

+++ b/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorage.php
@@ -1097,7 +1097,7 @@ protected function mapToStorageRecord(ContentEntityInterface $entity, $table_nam
+        $value = SqlContentEntityStorageSchema::castValue($definition->getSchema()['columns'][$column_name], $value);

@@ -1398,7 +1398,7 @@ protected function saveToDedicatedTables(ContentEntityInterface $entity, $update
+            $record[$column_name] = SqlContentEntityStorageSchema::castValue($attributes, $value);

+++ b/core/lib/Drupal/Core/Entity/Sql/SqlContentEntityStorageSchema.php
@@ -2099,7 +2099,7 @@ protected function getSharedTableFieldSchema(FieldStorageDefinitionInterface $st
+        $schema['fields'][$schema_field_name]['initial'] = SqlContentEntityStorageSchema::castValue($column_schema, $initial_value[$field_column_name]);

Do these mean we're loading SqlContentEntityStorageSchema on every request (~2300 LoC)? I'd rather have put this into the storage class itself to avoid that...