Deprecate Gateway.Writable in Kubo 0.19

[lidel]

Gateway.Writable is opt-in, enables very old legacy code that has no specs.

We plan to eventually create specs for onboarding data over HTTP POST and PUT (ipfs/specs#375) but it will not happen any time soon.

We are extracting things that have specs to the reference library (#8524), but the code behind Gateway.Writable should stay in Kubo (or be removed until we re-introduce something with a a spec).

Deprecation plan

1. Replace Gateway.Writable with optional Flag to remove it from the default config created by ipfs init.
2. See if we can do a civilized deprecation period and NOT break users in Kubo 0.19

• (soft deprecation) If possible, keep it working (move handlers for Gateway.Writable to kubo/core/corehttp/ for now)
  • If user has it enabled, display deprecation error during daemon start
• (hard removal) if there is no other way
  • If user has it enabled, display hard error informing about removal, asking them to remove Gateway.Writable from config

Examples of deprecation notices

• log error and refuse to start:

  kubo/cmd/ipfs/daemon.go

  Line 458 in 4e8271b

  log.Fatal("Private network does not work with Routing.Type=auto. Update your config to Routing.Type=dht (or none, and do manual peering)")

• log error, but continue:

  kubo/cmd/ipfs/daemon.go

  Line 406 in 4e8271b

  log.Error("Private networking (swarm.key / LIBP2P_FORCE_PNET) does not work with public HTTP IPNIs enabled by Routing.Type=auto. Kubo will use Routing.Type=dht instead. Update config to remove this message.")

Ref. https://github.com/ipfs/kubo/blob/master/docs/config.md#gatewaywritable

[lidel]

my vote is to not break people, unless this is a time sink to restore it after extraction.

remove from go-libipfs, but keep it working in Kubo. flag as deprecated like we did for object APIs so there is no expectation of it being around in 0.20. This gateway has been around for 5-8 years, people need at least 1 release of heads-up.