Expose RuntimeExecutor on ReactContext (#56987)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56987

Add `ReactContext.getRuntimeExecutor()` so callers can obtain the `RuntimeExecutor` without reaching into `CatalystInstance`. The bridge subclass forwards to `CatalystInstance.getRuntimeExecutor()`; the bridgeless subclass forwards to `ReactHost.runtimeExecutor`; `ThemedReactContext` proxies to its wrapped `ReactApplicationContext`. Previously, callers that needed the runtime executor had to reach through `catalystInstance.getRuntimeExecutor()`, which is bridge-only and required a `NoGetCatalystInstance` lint suppression. The new accessor works uniformly in both bridged and bridgeless modes.

Changelog:
[Android][Added] - Add `ReactContext.getRuntimeExecutor()`

Reviewed By: mdvacca

Differential Revision: D106647233

fbshipit-source-id: 88dcf55df5b72888ccf48ce30e33beeeb20e6368