devel/ectd35: Add version 3.5 of etcd

The current implementation of the targets could possibly be improved
by moving some of the bits into go.mk.

Sponsored by: Entersekt (previously Modirum MDpay)
Co-authored-by: Xavier Beaudouin <kiwi@FreeBSD.org>
Differential Revision: https://reviews.freebsd.org/D50164