feat(gateway): add DisableHTMLErrors option

[hacdias]

This adds a new option DisableHTMLErrors to gateway.Config. If this is true, then the errors will always be returned as text/plain with the raw error message. This will need to be bubbled up to Kubo, which will then allow ipfs.io to have a better insight into the error messages.

This also allows users of boxo/gateway library to do white-label hosting, without IPFS branding on error pages.

cc @cewood @mcamou

[codecov]

Codecov Report

Merging #463 (3acb593) into main (0d10169) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #463      +/-   ##
==========================================
- Coverage   66.41%   66.32%   -0.09%     
==========================================
  Files         207      204       -3     
  Lines       24692    24621      -71     
==========================================
- Hits        16398    16329      -69     
+ Misses       6868     6862       -6     
- Partials     1426     1430       +4     

Files Changed	Coverage Δ
gateway/gateway.go	93.75% <ø> (ø)
gateway/errors.go	86.84% <100.00%> (ø)

... and 13 files with indirect coverage changes

[lidel]

@hacdias I think this is a sensible flag to have here and in Kubo(no matter if this is used by bifrost infra or not).
HTML Error pages include IPFS brand and links to project docs, and users may want to disable that to have whitelabel hosting setup.

Just rename it, so it is more clear what it does, either:

• ForcePlainTextErrors
• DisableHTMLErrors ← imo preferable, does what it says, does not corner us into returning plain text error forever (we can start returning JSON instead, and this won't break or look awkward)

[lidel]

This feature is now nice and clear, thanks! ❤️

Mind bubbling up to Kubo?