Allow the access token to be pulled from settings

Problem/Motivation

The instance access token is currently stored in the state API. This means that the instance key isn't being exported into configuration.

We can improve this by allowing the instance key to be stored in the settings.php file. This would allow deployments to happen without needing to manually add in the instance key.

Steps to reproduce

n/a

Proposed resolution

Add something like the following to the getAccessToken() method in the MastodonApiInstance service class.

    return $this->state->get('mastodon_api.' . $this->id() . '.mastodon_access_token') ?? $this->settings->get('mastodon_api.' . $this->id() . '.mastodon_access_token', '');

Also, the 'state' and 'settings' services can be injected into this class. There is no need to use an embedded \Drupal::state() callback.
Scratch that, this is a config entity class.

Remaining tasks

Add a test for this. The settings service can be mocked quite easily.