Define a D-Bus API with Portal conventions

XDG Portals have their own conventions, for example, they avoid using signals for long-running async processes.

I think that the portal implementation should be separate from the credential service D-Bus API itself, so we should basically create a new API that maps to calls on the credential service. (For one reason, this moves a lot of the complexity out of the main xdg-desktop-portal implementation, and allows using other languages like Rust without having to touch that repo's build system.)

Review the Portal Common Conventions for more guidance.

Beginne damit, die im Issue verlinkten XDG Portal Common Conventions zu lesen, und vergleiche sie anschließend mit der bestehenden Credential-Service-D-Bus-API. Definiere die separate portal-facing API und dokumentiere, wie ihre Aufrufe auf den Credential Service abgebildet werden, einschließlich des erforderlichen Verhaltens asynchroner Operationen.