Introduce ReadOnlyDistributionQueue API

We currently export an SPI with a fully editable queue, DistributionQueue.

Some distribution queue implementation are read-only meaning that they don't provide the ability to add/remove items. Example of such read-only queue is an implementation on top of an append only messaging service.

In this issue, I suggest to introduce add and export the ReadOnlyDistributionQueue interface. The existing read-only method signatures from DistributionQueue would be moved from DistributionQueue to ReadOnlyDistributionQueue. DistributionQueue would extend from ReadOnlyDistributionQueue. This change is backward compatible.