Breaking postcard version updates cause unclear version mismatch error

[FelixMcFelix]

As seen in omicron#4414, upgrading the version of postcard from 0.x -> 1.x leads to mutual unintelligibility between OPTE 0.25 and 0.26. There is every possibility this could happen again in future: either via postcard 2.0, or we might choose another wire format.

We can probably:

• make this error clearer in the API surface (i.e., indicate in our Error impl that a DeserCmdErr may well be an API version mismatch). The two main consumers here are opteadm and sled-agent, most likely.
• move one or more fields outside of the OpteCmdIoctl struct: api_version being the obvious one, or we could prepend a separate wire-format version. This would still cause another messy break in parse behaviour, so we'd need to combine it with some clarifiaction of the failure mode.