esp32: Add support for mDNS queries and responder.

They are both enabled by default, but can be disabled by defining
MICROPY_HW_ENABLE_MDNS_QUERIES and/or MICROPY_HW_ENABLE_MDNS_RESPONDER to
0.  The hostname for the responder is currently taken from
tcpip_adapter_get_hostname() but should eventually be configurable.