Modelling Elster authentication and verification flow

Problem:

• Elster is a key user identification function for BundID (German and otherwise).
• It is still unclear at which stage of the user creation, authentication and verification process the BundID components should communicate with the Matrix BE (including MXID and Matrix security key).

Goal:

Clarify at which stage of the Elster account creation, authentication and verification process the Matrix components should interact with the available BundID components.

ACs:

• Using a (structurizr) sequence diagram, describe the step-by-step process for creating a user account and logging in with an ELSTER certificate.
• Document the diagram in this ticket and in the solution architecture repo: https://gitlab.opencode.de/fitko/matrix-g2x/solution-architecture

Notes/Resources:

• The best would be to get the documentation directly from the manufacturer.
• Related GitLab ticket: https://gitlab.opencode.de/fitko/matrix-g2x/planning/-/issues/36