Replace `PartialBatchSelector` with aggregation job extensions

[jcjones]

ietf-wg-ppm/draft-ietf-ppm-dap#762

AggregationJobInitializeReq (messages/src/lib.rs) needs AggregationJobExtension extensions<0..2^16-1> in place of the partial batch
selector.

Requires a new extension type and registry, a leader_selected_batch_id extension carrying the BatchID, strictly-increasing ordering, and Helper handling:

unknown type --> fail job unsupportedExtension; out of order -->  invalidMessage
leader-selected batch mode with the extension absent --> invalidMessage.