feat: use already existing microG account if possible

Description

Integrate microG login in AppLounge. Following MR is coming to reorganize the login architecture: !667 (merged)

The UX workflow is described here: https://gitlab.e.foundation/e/os/backlog/-/issues/3939#note_786829

Screenshots

Technical details

Tests

Issues

https://gitlab.e.foundation/e/os/backlog/-/issues/3939

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