config: Flag to disable Bitswap client and server

[lidel]

Kubo needs an explicit configuration Flag for controlling if Bitswap protocol is enabled, and also if we enabled only Client or also Client+Server.

Proposed feature

Add top level Bitswap to the config with two flags

• Bitswap.Enabled controls both client and server (Flag, enabled by default)
• Bitswap.ServerEnabled controls if node responds to WANTs (Flag, depends on Bitswap.Enabled, enabled by default)
• basic regression tests in kubo/test/cli to ensure flags truly disable it

Notes:

• we have Internal.Bitswap but that is for super low level knobs. we keep them there.
• we have some code prior art in rainbow/setup_bitswap.go where Bitswap server is opt-in
• when in doubt, follow convention we already have for things like AutoNAT, Relays, AutoTLS.

Use cases

• Bitswap.Enabled=false
  • Bootstrappers, Relays, libp2p proxies and various other pieces of infra don't need bitswap at all.
  • HTTP-only low-power web hosting that already runs with Reprovide.Interval=0 and Gateway.NoFetch=true and uses dag import for populating data.
    • This will increase in utility and priority as HTTP retrieval (bitswap: opportunistic block fetch over /http* boxo#608, HTTP retrieval proposal boxo#747) lands in boxo
• Bitswap.ServerEnabled=false
  • The main user of this would be IPFS Desktop. There would be an option to disable Server.
  • Rationale: In some jurisdictions fetching content is legal, but providing the same content to others may no longer be legal.
• Simplify user onboarding story
  • Ability to turn bitswap off will save many human hours across ecosystem. Instead of telling people to build custom binaries and maintaining them, already existing orchestration and docker images for Kubo can be used for tasks like HTTP provider, Relay or a Bootstrapper.