refactor(sdk): Put ruma-federation-api dependency behind a feature
In theory clients shouldn't make requests to the server-server API. A
way to work around it for this specific case would be to implement
MSC4383.
In the meantime, clients that don't want to use
`Client::server_vendor_info()` won't have to build the extra
dependencies added by ruma-federation-api.
The feature is enabled for the bindings, so it isn't a breaking change
for matrix-sdk-ffi.
Signed-off-by: Kévin Commaille <zecakeh@tedomum.fr>