added some metadata mock for frontend, fixed some tests

...rvice/src/main/java/at/tuwien/endpoints/FileEndpoint.java

...e/src/main/java/at/tuwien/endpoints/MetadataEndpoint.java

...st/java/at/tuwien/service/FileServiceIntegrationTest.java

.../src/test/java/at/tuwien/service/FileServiceUnitTest.java

...ava/at/tuwien/service/MetadataServiceIntegrationTest.java

.../test/java/at/tuwien/service/MetadataServiceUnitTest.java

.../java/at/tuwien/exception/ZenodoUnavailableException.java

...services/src/main/java/at/tuwien/service/FileService.java

...ices/src/main/java/at/tuwien/service/MetadataService.java

...es/src/main/java/at/tuwien/service/ZenodoFileService.java

...rc/main/java/at/tuwien/service/ZenodoMetadataService.java

fda-ui/components/QueryList.vue

fda-ui/layouts/default.vue

...ges/databases/_database_id/queries/_query_id/metadata.vue