Collect Exposure Risk Metadata (EXPOSUREAPP-4921) (#2255)

* added implementation for ExposureRiskMetadataDonor,

the de.rki.coronawarnapp.server.protocols.internal.ppdd.PpaData namespaces is populated by protos that are not yet present but will be added soon

Signed-off-by: Kolya Opahle <k.opahle@sap.com>

* updated imports to match new proto root classes, (still missing from repo)

Signed-off-by: Kolya Opahle <k.opahle@sap.com>

* switched to kotlin only solution for proto <-> settings string conversion

Signed-off-by: Kolya Opahle <k.opahle@sap.com>

* added unit test for exposure risk metadata serialisation

Signed-off-by: Kolya Opahle <k.opahle@sap.com>

* Added error handling to previousExposureRiskMetadata
Added test that checks for changes in ExposureRiskMetadata serialization

Signed-off-by: Kolya Opahle <k.opahle@sap.com>