Add an extension to set account password

[emersion]

This is a somewhat common operation users might want to perform. Allow clients to expose a proper UI for this.

Complications:

• Some servers might want clients to send the old password
• Some servers might want to send an authorization code via e.g. email

Prior art:

• https://lists.sr.ht/~emersion/soju-dev/patches/32827
• https://spec.matrix.org/v1.2/client-server-api/#post_matrixclientv3accountpassword