Breaking `postcard` version updates cause unclear version mismatch error

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.

Start by reviewing the OpteCmdIoctl structure, the DeserCmdErr and Error implementations, and how opteadm and sled-agent consume these errors. Determine how version mismatches currently fail and define a clearer API failure mode, with tests or consumer behavior demonstrating that the mismatch is identifiable.