docs: add example of gateway that proxies to ?format=raw

[hacdias]

Closes #142. Depends on #147.

Since this is largely based on #147, I'm basing this on top of that. I reorganized the functions so I could re-use them. Part of the fetcher code is mostly based on https://github.com/filecoin-saturn/caboose.

I just have to see how I can pass-through DNSLink and IPNS requests for resolution.

[hacdias]

Feedback welcome @lidel @aschmahmann. I'm not sure to which point IPNS is within the scope of this PR. We could use the raw keys, but we would still have to deal with DNSLink (#149).

[codecov]

Codecov Report

Merging #151 (47519ff) into main (302b279) will decrease coverage by 0.28%.
The diff coverage is 12.22%.

❗ Current head 47519ff differs from pull request most recent head 4dec0c5. Consider uploading reports for the commit 4dec0c5 to get more accurate results

@@            Coverage Diff             @@
##             main     #151      +/-   ##
==========================================
- Coverage   18.50%   18.23%   -0.28%     
==========================================
  Files          96       99       +3     
  Lines       10337    10537     +200     
==========================================
+ Hits         1913     1921       +8     
- Misses       8152     8342     +190     
- Partials      272      274       +2     

Impacted Files	Coverage Δ
examples/gateway/car/main.go	14.63% <0.00%> (ø)
examples/gateway/proxy/blockstore.go	0.00% <0.00%> (ø)
examples/gateway/proxy/main.go	0.00% <0.00%> (ø)
examples/gateway/proxy/routing.go	0.00% <0.00%> (ø)
examples/gateway/common/blocks.go	38.97% <58.33%> (ø)

[lidel]

While it is fine to have this example without IPNS/DNSLink, the bifrost production binary will need it – I've wrote some thoughts ipfs-inactive/bifrost-gateway#5.

@hacdias perhaps you could implement a PoC here that uses HTTP RPC at https://node[0-2].delegate.ipfs.io/api/v0/resolve for /ipns/ resolution and /api/v0/routing/get for IPNS raw records?

[hacdias]

@lidel I reworked and added IPNS resolution: DNSLink goes through normal DNS requests implemented in the namesys package, and then I added a proxy routing that does IPNS routing requests via ?format=ipns-record. Haven't added any tests yet, but it would be great to get some feedback.

[lidel]

Namesys with custom routing lgtm,
but for this to be an useful example it needs better error handling (details inline)

[lidel]

@hacdias lgtm! small asks below, then feel free to merge (ok to add subdomains later) 👍

[hacdias]

@lidel I updated the blockstore to add validation by default, which makes sense for this example, since we are proxying requests to a remote untrusted blockstore. I had missed that. I also added two smoke tests to ensure we get an HTTP error when the remote gives invalid content, and gives 200 and the right content when the content is indeed valid. I will merge once CI is green.