[Feature-request] Add controls for 'manage overrides' in symbols

_I am a designer and developer, at the beginning stages of updating our existing design system sketch library to a new one with reactsketchapp_

We currently use a lot of nested symbols where we set additional rules on the parent symbol to prevent overrides to some child symbols.

**Example: Form Label component**

This symbol is used within all our form components (inputs, dropdowns, checkboxes, etc.)
I want to use the "manage overrides" feature in sketch to limit what can be overridden on the symbol when nested in parent symbols.

The manage overrides setting in sketch:


Does this exist currently or can we add support for this functionality. Thanks in advance!

Start by tracing how react-sketchapp represents nested symbols and compare that behavior with Sketch's “Manage Overrides” controls shown in the issue. No source file or test is named, so first identify the symbol-handling entry points and existing coverage. Done means establishing whether override controls are supported and, if not, defining and implementing the requested controls with tests.

Written by the indexing model from the issue text.