Change v3 discovery url to endpoints so as to support failover

[ahrtr]

Part of 13624, and it's one of follow-ups to the PR 13635.

Currently the discovery url is just one endpoint. But actually it should be the same as the etcdctl, which means that it should be a list of endpoints. When one endpoint is down, the clientv3 can fail over to the next endpoint automatically.

cc @ptabor @serathius @spzala

[ptabor]

Thank you.

[ahrtr]

Resolved all your comments. The grpcProxy-test failure is because there are some leaked goroutines, and it should have nothing to do with this PR.

I manually executed the following command under the tests directory on the main branch, and I can also see the leaked goroutines,
'go' 'test' '-timeout=30m' '-tags' 'cluster_proxy' '--race=true' '--cpu=4' 'go.etcd.io/etcd/tests/v3/integration/clientv3' -v

I think we really need to refactor & improve the test framework & cases, and I see @serathius has already started the work. I may also spend some time on it later.

cc @serathius @ptabor @spzala

[ahrtr]

I just retriggered the test by submitted an empty --amend commit, all tests are green now. PTAL @ptabor @serathius @spzala