Support Username for connect to redis

[wrenix]

Description:

Current it is possible to configure dbid and password during a connection but not the username.
The username "default" is internal used.

[devonh]

I briefly looked into this and you are correct that Synapse is limited to using the default redis username.
I am not sure if it is possible to specify a username with our current usage of the txredisapi library.

If someone wanted to open a PR with changes to support this functionality, we would gladly accept it into Synapse.