[Feature] Simple template extension mechanism (#361)

Co-authored-by: Tim Lorsbach <tim@lorsba.ch>
Reviewed-on: enviPath/enviPy#361

epdb/template_registry.py

@@ -0,0 +1,17 @@
+from collections import defaultdict
+from threading import Lock
+
+_registry = defaultdict(list)
+_lock = Lock()
+
+
+def register_template(slot: str, template_name: str, *, order: int = 100):
+    item = (order, template_name)
+    with _lock:
+        if item not in _registry[slot]:
+            _registry[slot].append(item)
+            _registry[slot].sort(key=lambda x: x[0])
+
+
+def get_templates(slot: str):
+    return [template_name for _, template_name in _registry.get(slot, [])]