docs: add example of gateway backed by CAR file

[hacdias]

This PR adds:

• Example on how to build a Gateway backed by a CAR file and a few smoke tests that check directory traversing, as well as a DAG-JSON response. Closes gateway: create example that hosts a CAR file #141.
• CI that runs the tests from the examples based on docs(examples): add UnixFS file download over Bitswap #143.
• A readme for the examples based on docs(examples): add UnixFS file download over Bitswap #143.

Depends on #145.

[codecov]

Codecov Report

Merging #147 (691d1b9) into main (6399b73) will increase coverage by 0.21%.
The diff coverage is 32.69%.

@@            Coverage Diff             @@
##             main     #147      +/-   ##
==========================================
+ Coverage   18.28%   18.50%   +0.21%     
==========================================
  Files          94       96       +2     
  Lines       10181    10337     +156     
==========================================
+ Hits         1862     1913      +51     
- Misses       8058     8152      +94     
- Partials      261      272      +11     

Impacted Files	Coverage Δ
examples/gateway-car/api.go	31.37% <31.37%> (ø)
examples/gateway-car/main.go	35.18% <35.18%> (ø)

[hacdias]

@lidel @aschmahmann I'm happy to receive reviews here. The code I mentioned in the comment above for LsUnixFsDir is gigantic and I feel like this is something that I don't want to ask users to have to write. Perhaps wrapping it around a helper could be beneficial?

[aschmahmann]

Left some comments on the complexity with Ls. At a high level though, this PR is a little difficult to merge with no tests since we have no idea if it's correct or not 🙃 (e.g. issues with go-ipfs-files, or directory listing).

You could add a couple smoke tests in a CAR for now and then move onto #142 which IIUC should be roughly testable using sharness tests. Hopefully the code from #142 and here should be almost entirely shared and could make its way into a helper function to make this easier and better tested.

Later as we do more refactoring we can attempt to make most of the complexity here go away with better types that are specific to what the gateway code currently needs.

[hacdias]

@aschmahmann thanks for the review. I have tackled the comments and massively simplified the example. I will work on adding some smoke tests.

(...) helper function to make this easier and better tested. (...)

I'm almost entirely sure that the only difference between this PR and #142 will be the block service. It is quite trivial to have a Gateway API implementation given a block service that conforms to the interface.

[aschmahmann]

Generally LGTM. We've got some basic tests and hopefully we'll get the main machinery better tested in #142. Approving to remove my prior "Request changes" since it looks good and because I might not be available later today to approve.

[hacdias]

@aschmahmann I'm just adding CI for the tests still :)

[lidel]

lgtm, if you address small UX suggestions below, fine to merge

It may be useful to update this PR after #149 is done, and then expose subdomain on cid.ipfs.localhost:port but also ok to merge as-is for now

[hacdias]

@lidel I made the updates you requested and added a note to #149 so I don't forget to update this example once I move the DNSLink and subdomain code here. Regarding roots, I print all roots, whether there's 0 or 10. I will merge once CI is green.