Add static peer discovery to the ProxyAdminService config

[liam-lowe]

Fourth in the stack of proxy admin service config PRs (see stacked PR!) - a static peer discovery provider, for when there's no DNS.

static is a fixed list of host:port, one per sibling, (including this pod's own address).

proxyAdmin:
  peer:
    listenAddress: "0.0.0.0:6062"
    allowInsecure: true
    discovery:
      provider: static
      static:
        addresses:
          - "10.0.0.1:6062"
          - "10.0.0.2:6062"