Add token selecting API

Sometimes an app might want a user to specify a token they want to use. For example:
- An exchange might want to know what token a user wants to sell.
- A bounty app might want to know what token the user wants to offer as the fee.

Many tokens may not be auto-detected by MetaMask (especially in a very pluralistic token future), and so an API for a site to ask for a token, and for a user to reply from their current token list would be useful.

Sample API pseudo-code:
```javascript
const tokenDetails = await ethereum.send({
method: 'wallet_requestToken',
params: ['Please choose a token to post a bounty with.'], // The declared usage
})
```
The user would then be shown a menu of their current tokens, and asked to choose one, with the provided text as prompt. That token's address and metadata would be returned to the app as `tokenDetails`.

The issue names no files, tests, or implementation entry points. Start by locating the extension's provider API handling and token-list UI, then compare the proposed wallet_requestToken flow with existing wallet methods; done means a site can request a token with a prompt and receive its address and metadata after user selection.

Written by the indexing model from the issue text.