Fix inconsistent persistDeviceCode usage

[hafezdivandari]

On "Device Authorization" grant, the \League\OAuth2\Server\Repositories\DeviceCodeRepositoryInterface::persistDeviceCode() method has been used 3 times with 3 different purposes:

1. For persisting the new device code (when requesting a device code)
2. For updating the "User ID" and "User Approved" (when completing the device auth request)
3. For updating "Last polled at" (when responding to access token request)

This makes the implementation of this method too hard, as we have to manually determine that for which purpose this method has been called!

You may check the workaround I had to use to be able to implement this on Laravel Passport here.

This PR adds 2 new methods to fix this:

• \League\OAuth2\Server\Repositories\DeviceCodeRepositoryInterface::persistUser()
• \League\OAuth2\Server\Repositories\DeviceCodeRepositoryInterface::persistLastPolledAt()

[hafezdivandari]

@Sephster Would you please let me know if this PR is something that will be considered, or should we keep the current workaround in the implementation?

I could also use upsert for this, but I believe it's safer not to update all attributes each time.

[hafezdivandari]

Never mind! I've used upsert instead at laravel/passport@eca145a