Support arbitrary user-defined smp payloads

[JPHutchins]

Allow first customize fields of header with reasonable defaults for sequence, flags, and version.

smpmgr custom <op, group, command> [--sequence,--flags,--version] <cbor object>

Example

smpmgr custom 0 128 0 `{hello: "world", answer: 42, 1: 234}`

Response would be a simple header and cbor decode without any validation. Would not be able to use Header type for decoding since it validates.