feat: add proper optional-dependencies for each modules (#75)

src/fastapi_toolsets/_imports.py

@@ -0,0 +1,9 @@
+"""Optional dependency helpers."""
+
+
+def require_extra(package: str, extra: str) -> None:
+    """Raise *ImportError* with an actionable install instruction."""
+    raise ImportError(
+        f"'{package}' is required to use this module. "
+        f"Install it with: pip install fastapi-toolsets[{extra}]"
+    )