RN: Enforce `flow-typed` Signatures

Summary:
Creates a new `valid-flow-typed-signature` ESLint rule that validates the `flow-typed` signatures at the top of each file.

This lint rule will discourage contributors from locally forking the `flow-typed` definitions. Instead, any changes should be submitted as patches to the upstream definition in: https://github.com/flow-typed/flow-typed

Changelog:
[Internal]

Reviewed By: rickhanlonii

Differential Revision: D39868721

fbshipit-source-id: e5e3ffe7568dbe52c9b598b53110b0fcbcad3e38