[Conformance] add another endpoint to add with sync integration

[mhutchinson]

The current conformance suite has an /add endpoint that adds a given leaf and returns only the sequence number assigned.

A mode we have support for in Tessera, and that we want to encourage people to use, is to allow clients to add leaves and the personality response to include an offline inclusion proof (checkpoint, leaf index, inclusion proof).

Suggestion

Conformance binaries expose a new endpoint /addSync that takes the same request format, but blocks using the Awaiter until the checkpoint is available, and then returns the bundle information as above. We'll need to determine an encoding, but how hard can that be?