Provide configuration schema and rename config to ai_seo.settings

I have added the config folder with default settings and schema.
I have renamed ai_seo.configuration to ai_seo.settings so it follows the pattern of how normally modules name its config object.
I have made it possible to use the default AI provider and model even if no model is selected in ai_seo settings. So in case that you have already selected the default provider and model for chat operation type, there is no need to set it in ai_seo settings, unless you want to have different one just for this task. This way it is easier to start evaluating/working with ai seo functionality, as there is no need for additional step before you can start using it.

Please review MR.