Parse user given attribute events

Summary:
MSC3865 gives a way to store custom attributes in account data. This commit adds support to parse and set these attributes.

Type: add

CCBUG: https://iron.lily-is.land/T77

Test Plan: Verify that unit tests pass.

Reviewers: O1 the Kazv Project, nannanko

Reviewed By: O1 the Kazv Project, nannanko

Differential Revision: https://iron.lily-is.land/D37