feat: parse and validate FIT containers

app/fit/models.py

@@ -0,0 +1,16 @@
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class FieldDefinition:
+    num: int
+    size: int
+    base_type: int
+
+
+@dataclass(frozen=True)
+class LocalDefinition:
+    global_message_num: int
+    endian: str
+    fields: tuple[FieldDefinition, ...]
+    developer_field_size: int