Add SIG etcd leadership roles

[jmhbnz]

Recently SIG etcd was established, with two tech leads and chair roles.

As discussed and agreed through maintainer majority in etcd-io/etcd#16794 (comment) this pull request:

1. Renames the etcd project maintainers source of truth to our newly added OWNERS file.
2. Adds a separate section under etcd for SIG etcd leadership roles .

cc @serathius, @ahrtr, @wenjiaswe, @jberkus

[amye]

So, you can rename Maintainers to Owners, that's fine - but duplicate entries are going to throw things off. Can we consolidate?

[jmhbnz]

So, you can rename Maintainers to Owners, that's fine - but duplicate entries are going to throw things off. Can we consolidate?

Thanks @amye - I've consolidated the entries. For some context, I'm currently the only SIG etcd lead that isn't currently also a code approver specified in OWNERS (currently reviewer) so I've retained the second category with just myself under it. Over the lifecycle of the SIG and project moving forward it is possible future co-chairs could also be in the same category of not being code owners, but still needed elevated permissions within the project.