graphql: Apply authorization on /_search endpoint

- AccessControlContext now resides in a separate package from where it can be imported,
along with the contextKey that will be used to set and retrieve this context value.

- AccessControlContext has a new field called Username, that will be of use for future
implementations in graphQL resolvers.

- GlobalSearch resolver now uses this context to filter repos available to the logged user.

- moved logic for uploading images in tests so that it can be used in every package

- tests were added for multiple request scenarios, when zot-server requires authz
on specific repos

- added tests with injected errors for extended coverage

- added tests for status code error injection utilities

Closes https://github.com/project-zot/zot/issues/615

Signed-off-by: Alex Stan <alexandrustan96@yahoo.ro>

pkg/test/prod.go

@@ -11,6 +11,10 @@ func Ok(ok bool) bool {
 	return ok
 }
 
+func ErrStatusCode(statusCode int) int {
+	return statusCode
+}
+
 /**
  *
  * Failure injection infrastructure to cover hard-to-reach code paths (nop in production).