Add support for explicit, exclusive list of native modules for iOS RCTBridge implementation

[felixSchl]

Introduction

It appears currently impossible to create an RCTBridge instance that specifies an exclusive list of native modules to load. All native modules are registered program-wide via the accompanying macros and loaded every time, unconditionally. This is quite different from the Android implementation where we have total control over everything that gets loaded.

Details

The usecase is loading JS that should run in a restrictive, sandboxed environment with only a small subset of modules loaded which are carefully specified up front. Currently, on iOS, it is only possible to specific additional modules, but not the complete and exclusive list as far as I can tell.

Discussion points

• Consistent API and behavior across platforms
• Ability to sandbox JS on per-bridge instance

[kelset]

Maybe I'm misunderstanding, but isn't this something that the TurboModules approach fix by definition? #40