feat: Custom message files (#1992)

* feat: add get custom message text to admin api

* feat: read custom message texts from files

* feat: get languages in apis

* feat: get languages in apis

* feat: get languages in apis

* feat: pr feedback

* feat: docs

* feat: merge main

migrations/cockroach/V1.55__custom_text.sql

@@ -0,0 +1,14 @@
+CREATE TABLE notification.custom_texts (
+   aggregate_id TEXT,
+
+   creation_date TIMESTAMPTZ,
+   change_date TIMESTAMPTZ,
+   sequence BIGINT,
+
+   template TEXT,
+   language TEXT,
+   key TEXT,
+   text TEXT,
+
+   PRIMARY KEY (aggregate_id, template, key, language)
+);