Configuration of plugin options also through admin

[sargreal]

The current way to configure the API Token and if the free or paid API is used can only be configured in the config file or through environment variables.
Since there is no other way to set these, the application setup is currently cancelled if these are not set.

It would be very useful, to allow configuration through the admin panel, since then also not so techy people can set it up.

Inspiration: https://github.com/meilisearch/strapi-plugin-meilisearch#-add-credentials-

• Allow api token and free api configurations to be empty
• Configure a router, controller and service to set and retrieve settings from the plugin store
  • Do not allow changing the settings if the values were set through the plugin configuration / environment variables
  • Use values from plugin configuration if they are set
• Create configuration page to set/update these values
  • Grey out fields if values from plugin configuration are present
• Show meaningful error messages when the credentials are not set up yet

[sargreal]

Since this option will now be tied to the provider system, this should probably be implemented with #27