Support etcd auth(username and password)

[ahuigo]

Preflight Checklist

• [X] I have searched the issue tracker for an issue that matches the one I want to file, without success.

Problem Description

Viper doesn't support etcd auth.

Proposed Solution

1. Merge this PR: https://github.com/sagikazarmark/crypt/pull/306
2. In order to be compatible with the existing viper, how about putting the auth information in the endpoint? like this??

// old /remote/remote.go
case "etcd3":
    cm, err = crypt.NewStandardEtcdV3ConfigManager([]string{rp.Endpoint()})

// new
case "etcd3":
    var username,password string
    endpoint := rp.Endpoint() //"user:password@127.0.0.1:2379"
    eparts := strings.SplitN(endpoint, "@",2)
    if len(eparts) == 2{
        endpoint = eparts[1]
         userpass := strings.SplitN(eparts[0],":",2)
         username = userpass[0]
         password = userpass[1]
    }
    cm, err = crypt.NewStandardEtcdV3ConfigManagerRaw(goetcdv3.Config{
        Endpoints: []string(endpoint),
        Username: username,
        Password: password,
    })

Alternatives Considered

Add a new remote provider for etcd :

etcdConfig:= goetcdv3.Config{
    Endpoints: "127.0.0.1:2379"
    Username: "username",
    Password: "password",
}
viper.AddEtcd3RemoteProvider(etcdConfig, "/path")

Additional Information

No response

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