feat(p2p): support inbound-only peers without underlay addresses

[gacevicljubisa]

Checklist

• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
• I have filled out the description and linked the related issues.

Description

Allow peers without dialable underlay addresses (e.g., browsers, WebRTC
clients, NAT-restricted nodes) to establish connections and use protocols
over existing streams.

Changes:

• Remove empty underlay validation in bzz.ParseAddress and MarshalJSON
• Skip addressbook persistence for peers with empty underlays
• Add nil check in notifyReacherConnected to prevent reacher crashes
• Rename peerRegistry.underlays to overlayToPeerID for clarity

Inbound-only peers can participate in protocols (pricing, retrieval, pss)
but are excluded from Kademlia topology and hive gossip since they cannot
be dialed back.

Open API Spec Version Changes (if applicable)

Motivation and Context (Optional)

Related Issue (Optional)

Screenshots (if appropriate):

[janos]

Very nice, maybe just to add some tests for behaviour changes.

[janos]

Also, do not add peer without underlays to the addressbook in hive here https://github.com/ethersphere/bee/blob/master/pkg/hive/hive.go#L314.