refactor headers into a dedicated package

Signed-off-by: Jörn Friedrich Dreyer <jfd@butonic.de>

graph/pkg/service/v0/net/headers/headers.go

@@ -0,0 +1,9 @@
+package headers
+
+const (
+	// "github.com/cs3org/reva/internal/http/services/datagateway" is internal so we redeclare it here
+	// TokenTransportHeader holds the header key for the reva transfer token
+	TokenTransportHeader = "X-Reva-Transfer"
+	// IfModifiedSince is used to mimic/pass on caching headers when using grpc
+	IfModifiedSince = "If-Modified-Since"
+)