UNIX Domain Socket support added for connect in parallel to Consul UDS support in v1.10

### Proposal
Consul 1.10 is releasing UDS support for services and Connect. Support has been included as of 1.10beta2 release. I've been experimenting with manually registering services like MySQL running on a UDS instead of TCP but this would clearly be useful in Nomad as well.

Consul has added the service option `socket_path` as alternative to `port` or `ports`. Ergo it would be a great paring with Consul if Nomad's service descriptor could support that.

Example:
```
service {
name = "count-api"
port = "9001"

connect {
sidecar_service {}
}
}

service {
name = "mysql"
socket_path = "/run/mysqld.sock"

connect {
sidecar_service {}
}
}
```

Start with Nomad's service descriptor and the Consul integration described in the proposal, then trace how the existing port and ports options are registered. Compare that path with Consul 1.10's socket_path option and determine the required behavior for connect sidecar services. Done means a service descriptor can express socket_path and Nomad registers it correctly alongside existing TCP options.

Written by the indexing model from the issue text.