Add `apiKeys.validateApiKey` method (#1373)

## Description

We're introducing API keys to help developers provide secure
authentication to their APIs. This PR exposes the validate API key
endpoint, which is currently under development and looks like this:

> ### Validate API key
> `POST /api_keys/validations`
> 
> #### Parameters
> 
> * `value`: The value for an API key
> 
> #### Returns
> 
> * `api_key`: `api_key` | `null`

Here's what a call to this new method might look like in a developer's
application:

```node
workos.apiKeys.validateApiKey({ value })
// { apiKey: { ... } } or { apiKey: null }
```

## Documentation

This will require a docs change, which will come in a future PR.