Use Key module for API key management

Problem/Motivation

While the module does not display Brandfolder API keys on its config form, the keys are stored using Drupal's config system. While config files should be stored outside a site's web root, they are typically tracked in version control, and these keys will also be tracked by default. This is not optimal, security-wise.

Proposed resolution

Use the Key module to store Brandfolder API keys, so users can easily configure their preferred key storage method.

Remaining tasks

Update Brandfolder config form and all code that currently pulls keys from config.
@see https://www.drupal.org/docs/contributed-modules/key/developer-guide

User interface changes

Key selection options on Brandfolder config form.

API changes

None.

Data model changes

New key storage and retrieval, internally.