Support embedding assets from a subdir

[yurishkuro]

Part of #2749.

• add util function for altering http.FileSystem behavior
• refactor HotROD embedding to read from a subdir directly
• replace esc with embedding for the main UI placeholder

[codecov]

Codecov Report

Merging #2846 (aca2af9) into master (0a34e46) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2846      +/-   ##
==========================================
+ Coverage   95.94%   95.96%   +0.02%     
==========================================
  Files         221      222       +1     
  Lines        9689     9696       +7     
==========================================
+ Hits         9296     9305       +9     
+ Misses        324      323       -1     
+ Partials       69       68       -1     

Impacted Files	Coverage Δ
pkg/httpfs/prefixed.go	100.00% <100.00%> (ø)
cmd/query/app/static_handler.go	96.77% <0.00%> (+1.61%)	⬆️

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[yurishkuro]

@albertteoh fix for that nested dir

[albertteoh]

nice! Looking forward to removing use of esc altogether; I noticed it's been a common question/problem from folks trying to run/build the backend components because they haven't installed it or $GOPATH isn't in their $PATH.

[yurishkuro]

Indeed. And the gen_assets kept being regenerated with only difference being the timestamp, pretty annoying.

We need to merge #2822 before the last esc piece can be removed.