feat(gateway): subdomain and proxy gateway

cmd/ipfs/daemon.go

core/corehttp/corehttp.go

core/corehttp/gateway_handler.go

core/corehttp/gateway_test.go

core/corehttp/hostname.go

core/corehttp/hostname_test.go

core/corehttp/ipns_hostname.go

docs/config.md

docs/environment-variables.md

go.mod

go.sum

namesys/namesys.go

namesys/namesys_test.go

namesys/routing.go

test/sharness/t0111-gateway-writeable.sh

test/sharness/t0114-gateway-subdomains.sh

test/sharness/t0160-resolve.sh

test/sharness/t0184-http-proxy-over-p2p.sh