Support authentication via the Authorization: Bearer <token> header.

See [section 4.1 of the Matrix spec](https://spec.matrix.org/v1.6/client-server-api/#using-access-tokens). As far as I can tell from my experiments attempting to use the [NewClient func](https://pkg.go.dev/maunium.net/go/mautrix#NewClient) it does not seem like setting the `accessToken` parameter causes the client to provide the Authorization header.