[FEATURE] v4/sync/etcd - add username/password options

[dbereza-machinify]

Is your feature request related to a problem? Please describe.
Currently, the v4/sync/etcd plugin does not allow passing in a username/password for RBAC authentication with etcd, either embedded in the endpoint URL or as separate custom options.

Describe the solution you'd like
The etcd client library expects the username/password for RBAC authentication passed in the config, which is being defined here:
https://github.com/go-micro/plugins/blob/0ed40ba1aacebe0d38d871a6272f78125bf68dbc/v4/sync/etcd/etcd.go#L174-L177

Currently, the username / password are not being defined here, and embedding the username/password into the endpoint URL does not cause the etcd client to parse them out and use them for RBAC auth.

As such, the username/password could be parsed out of the sync.Nodes sync option, which isn't super ideal, or new etcd-specific credential options could be created and stored/retrieved from the option context.

Additional context
N/A

[xpunch]

https://github.com/go-micro/plugins/blob/9711876631bf43de2a4da21a37bbb7353f846ae2/v4/registry/etcd/options.go#L20-L27
Hope this can be helpful.

[dbereza-machinify]

Yep yep, thanks!

I was hoping to use that same code once the Context option was added to the sync interface, for consistency :)