Grammar for account data types

[richvdh]

Link to problem area:

https://spec.matrix.org/v1.18/client-server-api/#client-config

Issue

No grammar is currently specified for the type of account data, so it's unclear for implementations what they have to support.

In particular:

• Which characters are allowed?
• What is the maximum length?
• Is a zero-length type permitted?

It seems like the common namespaced identifier grammar would be a good fit.

[richvdh]

For reference, Synapse (currently) allows any non-NUL unicode character, and allows zero-length strings, and has no maximum length other than that implied by the maximum URI length (in either Synapse or a reverse-proxy; Synapse's limit is 16K but reverse-proxies are typically lower).