GraphQL: consolidate response structures which are now used in zli and search tests

[andaaron]

Consolidate GraphQL response structures which are now in pkg/extensions/search/common/common_test.go and pkg/cli/service.go, move them to pkg/common, and reuse them in both ZLI and search tests. They are now more or less duplicate code.

This builds on top of the changes already done for #1342

[andaaron]

Current progress is on branch https://github.com/nicoldr/zot/tree/refactorStructs