test(gateway): remote-block-backend

[lidel]

This PR improves coverage of backends introduced in #594 and #587 by running it three times:

• local-block-backend (~ what Kubo does)
• remote-block-backend( ~ what bifrost-gateway did with GRAPH_BACKEND set to false, we want to port it to rainbow here)
• remote-car-backend( ~ what bifrost-gateway did with GRAPH_BACKEND set to true, we want to port it to rainbow here)

It is built on top of https://github.com/ipfs/boxo/pull/587 so merging there as soon CI is green here. We can refine things there.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 59.64%. Comparing base (548a2c5) to head (fa1b20b).

@@                     Coverage Diff                     @@
##           gateway-implementations     #600      +/-   ##
===========================================================
- Coverage                    59.71%   59.64%   -0.08%     
===========================================================
  Files                          238      238              
  Lines                        29810    29810              
===========================================================
- Hits                         17802    17781      -21     
- Misses                       10400    10415      +15     
- Partials                      1608     1614       +6     

Files	Coverage Δ
gateway/backend_car_fetcher.go	78.72% <ø> (ø)
gateway/blockstore.go	0.00% <ø> (ø)

... and 11 files with indirect coverage changes