Fixes#5902
## What this PR does
Keeps the existing `Client::send()` demo but extends it with:
- A doc comment on `get_profile` explaining the spec behaviour and the 401
condition on (Synapse's `require_auth_for_profile_requests`)
- A `get_profile_authenticated()` fallback using `account().fetch_user_profile()`
which internally uses `force_auth()`
Handle the changes to the Response of the get_profile endpoint. The
content of the response is private and fields must be accessed with
methods.
Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>
