Circuit Relay Integration

#### Type: Feature

#### Severity: Medium

#### Description:
[Circuit Relay](/libp2p/specs/tree/master/relay) provides us with the means to indirectly connect peers who cannot directly connect to each other, either because of NAT or because of protocol incompatibilities (eg: js interop).
The relay specification has been implemented in Go as a transport in [go-libp2p-circuit](/libp2p/go-libp2p-circuit).

#### Integration approach:
Per discussion with @whyrusleeping:
For initial deployment, the relay functionality should be purely manual: peers would have to manually connect to relays and explicitly open connections to each other by directly using `/p2p-circuit` addresses with `ipfs swarm connect`.
The relay transport should be enabled by default, with hop relay functionality an opt-in feature.
No relay addresses should be announced by the host through the DHT, until we better understand the heuristics for automatically using them

Start by reading the Circuit Relay specification and the Go implementation in go-libp2p-circuit, then inspect how Kubo exposes `ipfs swarm connect`. Done means relay transport is enabled by default, hop relay functionality remains opt-in, manual `/p2p-circuit` connections work, and relay addresses are not announced through the DHT.

Written by the indexing model from the issue text.