Static DNS lookups, in addition to explicitly targeting a datacenter,
can target a cluster peer. This was added in 95dc0c7b30 but didn't make the documentation.
The driving function for the change is `parseLocality` here: 0b1299c28d/agent/dns_oss.go (L25)
The biggest change in this is to adjust the standard lookup syntax to tie
`.<datacenter>` to `.dc` as required-together, and to append in the similar `.<cluster-peer>.peer` optional argument, both to A record and SRV record lookups.
Co-authored-by: David Yu <dyu@hashicorp.com>
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