Add entry_before_update and entry_before_add hooks (#7977)

Discussion: https://github.com/FreshRSS/FreshRSS/discussions/7973

Changes proposed in this pull request:

- Add new extension hook "entry_before_add"
- Add new extension hook "entry_before_update"

How to test the feature manually:

1. Create extension that uses the hooks and confirm they are invoked correctly.

Extension to use for testing
https://github.com/rnkln/freshrss-xExtension-Discord/pull/2

app/Controllers/feedController.php

@@ -639,6 +639,12 @@ class FreshRSS_feed_Controller extends FreshRSS_ActionController {
 							// If the entry has changed, there is a good chance for the full content to have changed as well.
 							$entry->loadCompleteContent(true);
 
+							$entry = Minz_ExtensionManager::callHook('entry_before_update', $entry);
+							if (!($entry instanceof FreshRSS_Entry)) {
+								// An extension has returned a null value, there is nothing to insert.
+								continue;
+							}
+
 							$entryDAO->updateEntry($entry->toArray());
 						}
 					} else {
@@ -672,6 +678,12 @@ class FreshRSS_feed_Controller extends FreshRSS_ActionController {
 							$feed->pubSubHubbubError(true);
 						}
 
+						$entry = Minz_ExtensionManager::callHook('entry_before_add', $entry);
+						if (!($entry instanceof FreshRSS_Entry)) {
+							// An extension has returned a null value, there is nothing to insert.
+							continue;
+						}
+
 						if ($entryDAO->addEntry($entry->toArray(), true)) {
 							$nbNewArticles++;
 						}