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/requestcontext/context.go

@@ -0,0 +1,18 @@
+package requestcontext
+
+type Key int
+
+// request-local context key.
+var authzCtxKey = Key(0) // nolint: gochecknoglobals
+
+// pointer needed for use in context.WithValue.
+func GetContextKey() *Key {
+	return &authzCtxKey
+}
+
+// AccessControlContext context passed down to http.Handlers.
+type AccessControlContext struct {
+	GlobPatterns map[string]bool
+	IsAdmin      bool
+	Username     string
+}