Add support for tokens in room shares

Tokens will be used to give access to a share to guests in public rooms.
Although the token itself is created in the provider of room shares and
no changes are needed for that, due to the code structure it is
necessary to explicitly call the provider from the manager when getting
a room share by token.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>