Add custom Ansible filter plugin get_category_entries

This commit introduces a new Ansible filter plugin named
'get_category_entries', which returns all role names under the
roles/ directory that start with a given prefix.

Additionally, unit tests (unittest framework) have been added under
tests/unit/filterplugins/ to ensure correct behavior, including:

- Returns empty list when roles/ directory is missing
- Correctly filters and sorts by prefix
- Ignores non-directory entries
- Supports custom roles_path argument
- Returns all roles when prefix is empty

Reference: https://chatgpt.com/share/68a2f1ab-1fe8-800f-b22a-28c1c95802c2

filter_plugins/get_category_entries.py

@@ -0,0 +1,31 @@
+# Custom Ansible filter to get all role names under "roles/" with a given prefix.
+
+import os
+
+def get_category_entries(prefix, roles_path="roles"):
+    """
+    Returns a list of role names under the given roles_path
+    that start with the specified prefix.
+
+    :param prefix: String prefix to match role names.
+    :param roles_path: Path to the roles directory (default: 'roles').
+    :return: List of matching role names.
+    """
+    if not os.path.isdir(roles_path):
+        return []
+
+    roles = []
+    for entry in os.listdir(roles_path):
+        full_path = os.path.join(roles_path, entry)
+        if os.path.isdir(full_path) and entry.startswith(prefix):
+            roles.append(entry)
+
+    return sorted(roles)
+
+class FilterModule(object):
+    """ Custom filters for Ansible """
+
+    def filters(self):
+        return {
+            "get_category_entries": get_category_entries
+        }