RuntimeTarget refactor - Add this-scoped async executors to all Targets

[motiz88]

Summary:
Changelog: [Internal]

This diff

1. Provides all Targets with an executorFromThis() method, which can be used from within a Target to access a this-scoped main thread executor = a std::function that will execute a callback asynchronously iff the current Target isn't destroyed first.
2. Refactors how (all) Target objects are constructed and retained, from a plain constructor to static shared_ptr create(). This is because executorFromThis() relies internally on enable_shared_from_this plus two-phase construction to populate the executor.
3. Creates utilities for deriving scoped executors from other executors and shared_ptrs.

The concept is very much like RuntimeExecutor in reverse: the #1 use case is moving from the JS thread back to the main thread - where "main thread" is defined loosely as "anywhere it's legal to call methods on Target/Agent objects, access session state, etc". The actual dispatching mechanism is left up to the owner of each PageTarget object; for now we only have an iOS integration, where we use RCTExecuteOnMainQueue.

Coupling the ownership/lifetime semantics with task scheduling is helpful, because it avoids the footgun of accidentally/nondeterministically moving shared_ptrs (and destructors!) to a different thread/queue .

This stack

I'm refactoring the way the Runtime concept works in the modern CDP backend to bring it in line with the Page/Instance concepts.

Overall, this will let us:

• Integrate with engines that require us to instantiate a shared Target-like object (e.g. Hermes AsyncDebuggingAPI) in addition to an per-session Agent-like object.
• Access JSI in a CDP context (both at target setup/teardown time and during a CDP session) to implement our own engine-agnostic functionality (console interception, Runtime.addBinding, etc).
• Manage CDP execution contexts natively in RN, and (down the line) enable first-class debugging support for multiple Runtimes in an Instance.

The core diffs in this stack:

• ~~Introduce a RuntimeTarget class similar to {Page,Instance}Target. ~~
• ~~Make runtime registration explicit (InstanceTarget::registerRuntime similar to PageTarget::registerInstance). ~~
• Rename the existing RuntimeAgent interface to RuntimeAgentDelegate.
• Create a new concrete RuntimeAgent class similar to {Page,Instance}Agent.
• Provide RuntimeTarget and RuntimeAgent with primitives for safe JSI access, namely a RuntimeExecutor for scheduling work on the JS thread.
  • Provide RuntimeTarget with mechanism for scheduling work on the "main" thread from the JS thread, for when we need to do more than just send a CDP message (which we can already do with the thread-safe FrontendChannel) in response to a JS event. ← This diff

Architecture diagrams

Before this stack:
https://pxl.cl/4h7m0

After this stack:
https://pxl.cl/4h7m7

Reviewed By: hoxyq

Differential Revision: D53356953

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