refactor(examples): proxy gateway to use exchange

[aschmahmann]

Using a blockstore isn't really the right interface here (e.g. Has doesn't really make any sense). The exchange.Interface or better the exchange.Fetcher interfaces are more appropriate and less confusing.

This also removes all the notImplemented errors. The functions not implemented are essentially optional, so descriptions as to what they are for have been left there but they otherwise should not cause issues.

Note: there seems to be some good cause here to simplify the examples (and make them more production-like) by adding:

• An implementation of the exchange interface that takes an exchange.Fetcher.
• An implementation of an LRU blockstore

Both of these are effectively implemented in bifrost-gateway (ipfs-inactive/bifrost-gateway#61). So once we're happy with how things pan out there we should consider porting implementations back to boxo.

Explicitly looking for feedback such as:

• Things to fix
• Comments to add/remove
• If we should hold off on merging this until some of the bifrost-gateway pieces have moved back here so the example is more production-like

Other feedback welcome too 😄

[codecov]

Codecov Report

Merging #229 (928df5d) into main (2835752) will increase coverage by 0.12%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
+ Coverage   48.02%   48.14%   +0.12%     
==========================================
  Files         269      269              
  Lines       32957    32962       +5     
==========================================
+ Hits        15827    15870      +43     
+ Misses      15454    15428      -26     
+ Partials     1676     1664      -12     

Impacted Files	Coverage Δ
examples/gateway/proxy/main.go	0.00% <0.00%> (ø)
examples/gateway/proxy/blockstore.go	49.31% <40.00%> (ø)

... and 9 files with indirect coverage changes

[hacdias]

LGTM. Tbf, when I wrote this for the first time I wasn't really aware of the other abstractions (like the exchange). It does indeed make more sense to use it.

[hacdias]

I rebased on top of #246 since I made quite a few changes to the examples there and we enabled more conformance tests, just to run more tests here.

[lidel]

Rebased since bunch of refactor had overlap with #246, now this PR is much smaller. lgtm