(core): Add app-global setting storage.

New `AppConfig` key-value table for arbitrary app config.
New `KeyValueData` setting data mixin.
New `AppSettings` settings group.
Attached `AppSettings` as `client.settings`.
Migrated sponsor settings to `AppSettings`.

bot/settings/base.py

@@ -1,3 +1,4 @@
+import json
 import discord
 from cmdClient.cmdClient import cmdClient
 from cmdClient.lib import SafeCancellation
@@ -459,5 +460,55 @@ class ListData:
                 cls._cache[id] = data
 
 
+class KeyValueData:
+    """
+    Mixin for settings implemented in a Key-Value table.
+    The underlying table should have a Unique constraint on the `(_id_column, _key_column)` pair.
+    """
+    _table_interface: Table = None
+
+    _id_column: str = None
+
+    _key_column: str = None
+
+    _value_column: str = None
+
+    _key: str = None
+
+    @classmethod
+    def _reader(cls, id: ..., **kwargs):
+        params = {
+            "select_columns": (cls._value_column, ),
+            cls._id_column: id,
+            cls._key_column: cls._key
+        }
+
+        row = cls._table_interface.select_one_where(**params)
+        data = row[cls._value_column] if row else None
+
+        if data is not None:
+            data = json.loads(data)
+
+        return data
+
+    @classmethod
+    def _writer(cls, id: ..., data: ..., **kwargs):
+        params = {
+            cls._id_column: id,
+            cls._key_column: cls._key
+        }
+        if data is not None:
+            values = {
+                cls._value_column: json.dumps(data)
+            }
+            cls._table_interface.upsert(
+                constraint=f"{cls._id_column}, {cls._key_column}",
+                **params,
+                **values
+            )
+        else:
+            cls._table_interface.delete_where(**params)
+
+
 class UserInputError(SafeCancellation):
     pass