Align codec interface to exclusively use packers

[StephenButtolph]

Why this should be merged

Because the interface doesn't include a Marshal function, it feels very weird to have an Unmarshal function.

How this works

Removes the Unmarshal function from the interface.

How this was tested

• Existing CI

[tsachiherman]

it would impact non-genericCodec unmarshalers as well. I don't know if we have any.
generally speaking, this would prevent serializing a series of objects into the same byte array in a sequence. This might be ok if we don't have any such usage.

[StephenButtolph]

it would impact non-genericCodec unmarshalers as well. I don't know if we have any. generally speaking, this would prevent serializing a series of objects into the same byte array in a sequence. This might be ok if we don't have any such usage.

Good point, we don't have any such usage. I think going forward this PR (and it's parent) standardize this a bit. After these PRs the manager enforces a codec version with no trailing data, the internal codecs don't know about the codec version and support reading from a sequence.