[red-knot] function signature representation (#14304)

## Summary

Add a typed representation of function signatures (parameters and return
type) and infer it correctly from a function.

Convert existing usage of function return types to use the signature
representation.

This does not yet add inferred types for parameters within function body
scopes based on the annotations, but it should be easy to add as a next
step.

Part of #14161 and #13693.

## Test Plan

Added tests.

crates/red_knot_python_semantic/resources/mdtest/call/function.md

@@ -19,6 +19,15 @@ async def get_int_async() -> int:
 reveal_type(get_int_async())  # revealed: @Todo
 ```
 
+## Generic
+
+```py
+def get_int[T]() -> int:
+    return 42
+
+reveal_type(get_int())  # revealed: int
+```
+
 ## Decorated
 
 ```py