Add access to metadata endpoint and use IDs in binary mode to reduce size

[garethpotter]

No description provided.

[martinjaeger]

I'm a bit wary to accept this PR, as it would consume almost all remaining two-byte reserved IDs. In addition to that, it's currently not documented in the specification.

Would it be possible to keep this application-specific? Or at least behind a Kconfig? What are your thoughts?

[garethpotter]

So, my initial reply was going to be along the lines of the fact that the metadata endpoint is already behind a Kconfig, and that because we use this a lot at Brill, the strings on the wire are needlessly verbose.

However, on thinking further, I realise that these aren't really ThingSet IDs in the traditional sense anyway: you can't pull 0x1A out and get a value. If they are anything they are record members (and I have thoughts about those anyway). As a consequence, they just end up being "logically"-named internally-defined keys, so perhaps the better thing to do would be rename the #defines so as not to create the impression that they are IDs.