feat: add message definition for encoded state

[domire8]

Description

• ROS message type for Protobuf messages #131

As described in the parent issue, it would be better to define our EncodedState explicitly and not as pure typedef. These changes accomplish that with the minimal amount of change. Two things:

1. I'm not sure if the layout field in the message definitions helps with serializing or anything (ROS internal) so I'm not sure if it's worth keeping
2. The typedef in modulo core is now a bit redundant. I could remove those and just change all the includes which have the same result with more time effort but is maybe a bit cleaner?

Review guidelines

Estimated Time of Review: 5 minutes

Checklist before merging:

• Confirm that the relevant changelog(s) are up-to-date in case of any user-facing changes

[LouisBrunner]

From what I understand, none of the usage have to change for this? Having a message with a single data is equivalent to what Uint8Array used to do? If so LGTM

[eeberhard]

Needs a changelog entry. API side, nothing changes because of the convenient typedef. Might still be ABI breaking though, not sure; it would be nice to be part of 5.0 proper, so I'm glad we have this release candidate.