Add User Management API support

[kengu]

Implement EventStoreUsersClient from

service Users {
	rpc Create (CreateReq) returns (CreateResp);
	rpc Update (UpdateReq) returns (UpdateResp);
	rpc Delete (DeleteReq) returns (DeleteResp);
	rpc Disable (DisableReq) returns (DisableResp);
	rpc Enable (EnableReq) returns (EnableResp);
	rpc Details (DetailsReq) returns (stream DetailsResp);
	rpc ChangePassword (ChangePasswordReq) returns (ChangePasswordResp);
	rpc ResetPassword (ResetPasswordReq) returns (ResetPasswordResp);
}

message CreateReq {
	Options options = 1;
	message Options {
		string login_name = 1;
		string password = 2;
		string full_name = 3;
		repeated string groups = 4;
	}
}

message CreateResp {

}

message UpdateReq {
	Options options = 1;
	message Options {
		string login_name = 1;
		string password = 2;
		string full_name = 3;
		repeated string groups = 4;
	}
}

message UpdateResp {

}

message DeleteReq {
	Options options = 1;
	message Options {
		string login_name = 1;
	}
}

message DeleteResp {

}

message EnableReq {
	Options options = 1;
	message Options {
		string login_name = 1;
	}
}

message EnableResp {

}

message DisableReq {
	Options options = 1;
	message Options {
		string login_name = 1;
	}
}

message DisableResp {
}

message DetailsReq {
	Options options = 1;
	message Options {
		string login_name = 1;
	}
}

message DetailsResp {
	UserDetails user_details = 1;
	message UserDetails {
		string login_name = 1;
		string full_name = 2;
		repeated string groups = 3;
		DateTime last_updated = 4;
		bool disabled = 5;

		message DateTime {
			int64 ticks_since_epoch = 1;
		}
	}
}

message ChangePasswordReq {
	Options options = 1;
	message Options {
		string login_name = 1;
		string current_password = 2;
		string new_password = 3;
	}
}

message ChangePasswordResp {
}

message ResetPasswordReq {
	Options options = 1;
	message Options {
		string login_name = 1;
		string new_password = 2;
	}
}

message ResetPasswordResp {
}

See EventStore.Client.UserManagement