Provider

Plugin interface for mail storage and transport backends.

Provider is the plugin descriptor structure that describes a mail storage or transport backend (or both). It is the first piece of information loaded from an external backend shared library and serves as the entry point for the plugin mechanism.

From the client's perspective, a provider descriptor gives enough information to present the user with the configuration questions required to set up the service, including the list of supported authentication mechanisms.

Key fields in the Provider structure: - `protocol`: The unique, lower-case URI scheme (e.g. `"imap"`). - `name`: A short human-readable name (e.g. `"IMAP"`). - `description`: A longer description of what the backend does. - `object_types`: The Store and/or Transport GTypes to instantiate for this backend. - `authtypes`: The list of supported authentication mechanisms. - `url_flags`: Flags describing which URI fields are required or optional.

Providers are loaded from shared libraries via provider_load and registered with the session using add_service. Once registered, Session uses the provider's `object_types` to instantiate the appropriate Service subclass when camel_session_get_service is called with a matching URI.