[swift] Add bindings for other functional reactive programming frameworks

[dhavalshreyas]

Audit areas of the API where we expose ReactiveSwift and build bindings for RxSwift, Combine.

Initial pass shows these areas:

Workflow

• WorkflowHost class:
  https://github.com/square/workflow/blob/8c10f6069915310600fe771f224d668000fadaa2/swift/Workflow/Sources/WorkflowHost.swift#L98

• Worker protocol:
  https://github.com/square/workflow/blob/8c10f6069915310600fe771f224d668000fadaa2/swift/Workflow/Sources/Worker.swift#L32

• RenderContext class:
  https://github.com/square/workflow/blob/8c10f6069915310600fe771f224d668000fadaa2/swift/Workflow/Sources/RenderContext.swift#L74

WorkflowUI

• ContainerViewController class
  https://github.com/square/workflow/blob/14f5c1e28c630254e5190ee37b8c3e38946c9717/swift/WorkflowUI/Sources/Container/ContainerViewController.swift#L28

WorkflowTesting

None