Request: Ethereum Provider API types declarations

## **Why?**

Reading MetaMask Docs - Getting Started to develop a DApp, i think that this guide maybe is uncomplete because, my actual situation developing a DApp with Angular, try to integrate a MetaMask Wallet connection its so confused (i understand that blockchain or descentralized app development its complicated); which leads me to do this implementation search for types of Ethereum Provider API.

At this point, this can be helpfull since DX approach, providing us a better intellisense help (via Typescript) to better development for begineers.

I think that its a better way import all types or typed code from a single source instead of declare it into each project + looking for types + copy/paste all this declarations.

### For example:

```ts
// On lib/module/dep/etc...
export interface RequestArguments {
method: string;
params?: unknown[] | object;
}

// On project
import { RequestArguments } from '@metamask/onboarding';

const myArgs: RequestArguments = { };
ethereum.request(args): Promise;
```

And according to Create a simple DApp tutorial, add situations like:

- Persistent data/session
- Practical examples for actions like connect/disconnect at same time that wallet
- etc...

I think that its a better way to do most complete initial implementation

P.D: In a future, possibly when my knowledge about this topic increse; i send a PR contributing. Apologies for my bad english

No files, tests, or entry points are named. First review the library's current public API and onboarding documentation, then define the scope for Ethereum Provider API declarations and the requested DApp examples; done means an agreed, complete implementation scope with corresponding published types and guidance.

Written by the indexing model from the issue text.