Make a marshallable interface/type constraint

- [ ] [Marshal](https://github.com/google/go-tpm/blob/50ce9737c31f0789c301c91ff0d246d6f18dd5a6/direct/tpm2/reflect.go#L127) takes interface{} so will panic if you accidentally pass something that is not marshallable.

- Possible improvements:

- Define an interface for types that can be `Marshalled` in a compact way that minimizes code bloat.

- [ ] Type annotations are a bit magic.

- Someone working on this code might get the `gotpm:` type annotations incorrect or slightly incorrect. It would be interesting to look into ways to make it more obvious what to do when introducing new TPM structures from the spec.

Start with direct/tpm2/reflect.go at the Marshal function around line 127, and inspect how interface{} values and gotpm: type annotations are used. Determine a concrete, agreed scope for a marshallable interface or clearer annotation validation; done should include a defined approach that avoids accidental panics and makes TPM structure annotations less error-prone.

Written by the indexing model from the issue text.