Create class types for common identifiers

[hikingc]

This will solve a lot of problems with Room, Events, Users and Aliases, will also uncouple validations from services and improve error handling (fail-fast).

These are meant for USER INPUT! Any server that sends malformed IDs are not the library problem.

Spec notes

[hikingc]

Done basic identifiers on 96d7b80
Some identifiers are still missing, not gonna mark it as solved until they all are mapped

[hikingc]

https://spec.matrix.org/v1.19/client-server-api/#matrix-content-mxc-uris

Implement for: https://spec.matrix.org/v1.19/client-server-api/#mroomavatar