[WIP] PoC: Use jsi::Value from prop directly

[hannojg]

Summary:

🚧 THIS PoC IS WORK IN PROGRESS

This PR highlights how we'd like to access the underlying jsi::Value directly to work with complex data types currently not supported for our Fabric Component Views.

The problem basically being that the jsi prop object we receive from JS in the FabricUIManager is converted to a RawProps type, which only allows us to access props via a .at method which returns a RawValue. This RawValue only supports folly::dynamic types (primitives, arrays and objects), to which we convert the original jsi::Value for every prop.

We'd like to pass complex types such as HostObjects or jsi::Objects with NativeState attached to our components.

The goal of this PoC PR is not to be merged to core but to discuss how we can make such a change happen.

Possible solutions

Expose jsi::Value and jsi::Runtime in RawProps

RawProps already holds the jsi::Object to the original prop object which we could simply expose to access the real underlying jsi::Value of a prop, which is what this PoC does:

https://github.com/hannojg/react-native/blob/e186024d43bb7f8ed70e73fe4ed334bc534eb9af/packages/react-native/ReactCommon/react/renderer/core/RawProps.h#L97-L101

and then when creating the props for our specific fabric component we can do:

https://github.com/hannojg/react-native/blob/e186024d43bb7f8ed70e73fe4ed334bc534eb9af/packages/rn-tester/ManualComponent/ManualFabricComponentView.mm#L27-L42

Have jsi::Value and jsi::Runtime in RawValue

We could have a pair of jsi::Value and jsi::Runtime in RawValue. Then for the prop implementation for a fabric native component view we could use this value during parsing to convert it to our desired complex type:

https://github.com/hannojg/react-native/blob/e186024d43bb7f8ed70e73fe4ed334bc534eb9af/packages/react-native/ReactCommon/react/renderer/core/RawValue.h#L75-L88

Problems:

• I found it difficult to figure out in RawPropParser to decide when we should parse to folly::dynamic or when to use jsi::Value during parsing (here we'd need to decide whether to use a folly::dynamic arg or jsi::Value)

Make RawProps hybrid / generic

The actual answer seems to be to make the RawPropsParser generic. A PoC explanation can be found here:
#44966 (comment)

Alternative path to RawProps

In the UIManager add a second way here for example:

react-native/packages/react-native/ReactCommon/react/renderer/uimanager/UIManagerBinding.cpp

Line 258 in 0d5c026

RawProps(runtime, arguments[3]),

Where based on some criteria we wouldn't create RawProps but an alternative wrapper (that gives us access to the original jsi types)

Problems:

• Not really a problem but it seems like we had to change a ton of places to account for that as RawProps are used / assumed in a lot of places

Changelog:

Test Plan:

[react-native-bot]

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