Bind outbound proxy to the IPv6 loopback

[alpeb]

In order for the proxy to support IPv6 traffic we need to bind the outbound proxy to the IPv6 loopback (::1), in addition to the existing binding to the IPv4 loopback (127.0.0.1).

Note, this is not necessary for inbound; on the proxy side we'll set LINKERD2_PROXY_INBOUND_LISTEN_ADDR (and the other *_LISTEN_ADDR) vars to the IPv6 wildcard address (::) which also works for IPv4.

This change adds the LINKERD2_PROXY_OUTBOUND_LISTEN_ADDRS var, which is supposed to be set as 127.0.0.1:4140,[::1]:4140 (maximum two entries). If not set, we default to the value in LINKERD2_PROXY_OUTBOUND_LISTEN_ADDR.

The ServerConfig got expanded with an optional addr_additional field, which should be populated with [::1]:4140. When set, we bind using BindWithOrigDst's new method bind_additional which leverages ServerConfig's InsertParam implementation for putting its addr_additional into addr for the regular binding logic to continue.

Update:
The field ServerConfig.addr is now of the new type DualListenAddr which holds a primary SocketAddr and an optional one.
The outbound listener is now bound via the DualBind implementation of Bind which also binds to the second SocketAddr if set, merging the two streams using Tokio's merge stream extension method.

[alpeb]

Ok I've rebased and pushed again using the suggested approach. I introduced a new DualBindWithOrigDst implementation of the Bind trait, used by the outbound listener which forwards two bind calls to BindWithOrigDst. Given we need to return two addresses, I was forced to expand the Bound type to be a triple, which introduced some extra baggage there where we don't require binding twice. Perhaps we can modify that type a bit to make this less ugly.
I also had to introduced an ExtractParam<ServerConfig, SocketAddr> implementation to make that DualBindWithOrigDst->BindWithOrigDst chain work; it didn't feel great so I'm all ears on suggestions.

[alpeb]

In my last push I added the BoundAddrs associated type to Bind to avoid having to deal with the triple everywhere. Also moved dual_bind as a submodule of listen, and other cleanups.

[olix0r]

This looks great to me! Thanks. Two small suggestions.