`KeysUploadRequest` has invalid `device_keys` value

[richvdh]

Synapse recently added some validation to /keys/upload (element-hq/synapse#17097). As a result of that, it turned out that matrix-bot-sdk was calling /keys/upload with device_keys: null.

Synapse has worked around this for now (element-hq/synapse#19023) but it remains non-spec-compliant behaviour on the client side which we need to fix. Synapse will begin enforcing this in a few months (element-hq/synapse#19030).

[richvdh]

Some historical notes:

• We previously had a related problem with the timeout param in /keys/query and /keys/claim. This was fixed in
  matrix-rust-sdk-crypto-nodejs, but that fix was not ported to matrix-sdk-crypto-wasm, meaning...
• We later rediscovered the same problem in matrix-sdk-crypto-wasm. That was then fixed, alongside a bug which caused device_keys to be malformatted in /keys/upload requests. Unfortunately, that fix to KeysUploadRequest was not ported to matrix-rust-sdk-crypto-nodejs, and so here we are.

As I wrote previously, it feels like we have a lot of complexity for serializing these two types, whereas all the information we need should be in the underlying Ruma types. Worse, we have two copies of that complexity, which keep getting out of sync.

Further: I don't understand how that request macro works. I think this contributes significantly to the problem here: it is virtually impossible to satisfy oneself by inspection that the macro implementation is correct. Personally, I would prefer to see manually-maintained TryFrom implementations, even if we have ~8 nearly-identical definitions, but at the very least, the macro needs some documentation explaining how to use it, and some comments explaining what it is doing.