[cdac] Publish data stream spec #99299

Add the specification doc for the cDAC data stream to docs/design in this repo

In #99936 we defined algorithmic contracts. In #100253 we defined a physical representation for the data descriptors.

This issue is about connecting those two things together and defining the physical representation of the data contract payload for a particular target runtime instance: the data descriptor together with the set of algorithmic contracts compatible with the target runtime instance.

We have a limited budget (<= 9 kilobytes, not tens of kilobytes) for this payload for release configurations.