gateway: remote backend implementations from bifrost-gateway

[lidel]

About gateway backends

Right now, boxo/gateway comes with only one implementation of local backend (BlocksBackend in gateway/blocks_backend.go).

Remote backends that follow https://specs.ipfs.tech/http-gateways/trustless-gateway/ as data transfer protocol were created in 2023 as part of Project Rhea, but are hard to discover and use outside the no longer actively maintained bifrost-gateway.

Proposed improvement

We should salvage that work and make these backends useful to boxo users by moving remote backend implementations from biforst-gateway to boxo/gateway

Namely:

• remote_blocks_backend.go (porting https://github.com/ipfs/bifrost-gateway/blob/main/blockstore_proxy.go)
• remote_car_backend.go (porting https://github.com/ipfs/bifrost-gateway/blob/main/lib/graph_gateway.go)

This will not only benefit boxo users, but also allow us to add these backends to rainbow (ipfs/rainbow#88) as alternative to libp2p one, allowing us to sunset and archive bifrost-gateway.

[lidel]

Triage notes:

• having "Fetch" abstraction based on this would be useful for refactoring/maintaining ipget or sole RPC in kubo (nothing to maintain + same behavior as gateway)