Add UID register to protocol

[bruno-f-cruz]

Summary

This PR adds a new register to the protocol called UID.

Motivation

We currently use a single U16 value to keep track of the serial numbers of devices. This is a rather small number and hardly future proof. Adhering on a pre-existing standard would make the implementation much more generic. For instance, it would allow one to co-opt pico's chip serial number (64bit) into this register.

Detailed Design

For the sake of backwards compatibility, we will add a new register instead of modifying the current R_SERIAL_NUMBER.
A new register will be added with the following specs:

Name: UID
Address: 16
Format: U8[16]
Access: Read-only

A deprecation warning was also added to the protocol regarding register R_SERIAL_NUMBER. For now, this register will duplicate the first two bytes of R_UID. On the next major release, the R_SERIAL_NUMBER will likely be removed and only R_UID supported.

Design Meetings

See further discussion in the following issues:

• Increase the size of the R_SERIAL_NUMBER to match 128bit UUID #31

[Poofjunior]

This looks good assuming it can be done without affecting R_DEVICE_NAME.

[bruno-f-cruz]

Use the address name UID instead of UUID
Suggest that the unique-ness of the ID should be unique within device type (i.e. whoami) but not necessarily across device types. This way people don't have to worry about clashes across devicetypes

[Poofjunior]

This is now implemented on the RP2040 Harp Core:
harp-tech/core.pico#30

[bruno-f-cruz]

• Re-iterate that its little-endian (https://github.com/harp-tech/protocol/blob/main/BinaryProtocol-8bit.md#introduction)
• Ensure that UID is consistent in the proposal (instead of UUID)