add etcd api v3 support

[jcarnu]

Hello,

I wrote a first try on etcd v3 api support in pgwatch2. I was wondering if there was a way to test it before submitting a PR and make some debugging with all the dockers things in the source code (sorry to bother you with that, maybe it worth asking you before going on writing my own test bench) ?

After a first quick reading it seems there's none.

Would you like me to submit a PR at first place before doing any testing actions (it compiles fine) so you could also make remarks on code and design ?

Roughly it's a simple hack of EtcdGetClusterMembers() and a simple duplication (bad) of extractEtcdScopeMembers()

Thanks.