SDK depends on Gitea

[jonasfranz]

go-sdk depends currently on code.gitea.io/gitea that implicates that all dependencies of gitea are needed to for go-sdk. We should consider fixing this since it will increase the size of binaries of tools using the sdk.

[lunny]

In fact, it only depends code.gitea.io/gitea/modules/structs, but go mod will require all the packages of gitea. Maybe we should put go.mod on code.gitea.io/gitea/modules/structs to fix this problem?