Implement snaps debugger using the iframe execution environment

The iframe execution environment enables us to create an outstanding debugging environment for snaps by integrating said environment with the OpenRPC toolchain. In its most basic form, it enables us to test if a snap will execute in SES, and whether its JSON-RPC API is working. In this form, the snap won't have access to the API MetaMask exposes to snaps in production, and we may have to stub those methods. However, with some additional work, we can actually add snaps to MetaMask and execute them inside an iframe created by the OpenRPC inspector playground.

- [x] Implement iframe execution environment OpenRPC inspector transport
- Done here: https://github.com/MetaMask/iframe-ee-openrpc-inspector-transport
- [ ] Do one or none of the following
- Create a mock MetaMask snaps provider to pass to plugins executing in the OpenRPC inspector
- Wire up the OpenRPC inspector as an actual execution environment that can be used for snaps added to MetaMask

Work on the execution environment itself is tracked in #24.