refactor and fix scoped context chat

[kyteinsky]

• added routes: /providers, /default-provider-key
• merge ScopedContextChatProvider to ContextChatProvider
• make use of ProviderService everywhere, which extends ProviderConfigService
• 'file' provider type is now files__default
• handle mimetype array parsing error in context_chat:scan command

[kyteinsky]

the integration tests would pass once the backend is merged (https://github.com/nextcloud/context_chat_backend/pull/22, having some CI issues there, not that it would pass but still) since this is a breaking change on /query