Add CORS support to HTTP endpoint

[bobcallaway]

This is blocking interactive use of the swagger tool against our production instance.

During this process, I noticed we were still passing some Grpc information in HTTP response headers so this also squelches those.

Signed-off-by: Bob Callaway bcallaway@google.com

[haydentherapper]

Error in test parse "passthrough:///:\x00\x00\x00\x00\x00\x00\x00": net/url: invalid control character in URL

[codecov-commenter]

Codecov Report

Merging #670 (cf60081) into main (21b6f72) will decrease coverage by 7.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #670      +/-   ##
==========================================
- Coverage   61.56%   54.44%   -7.12%     
==========================================
  Files          30       36       +6     
  Lines        1717     2228     +511     
==========================================
+ Hits         1057     1213     +156     
- Misses        575      923     +348     
- Partials       85       92       +7     

Impacted Files	Coverage Δ
cmd/app/http.go	53.96% <100.00%> (ø)
cmd/app/createca.go	19.49% <0.00%> (ø)
cmd/app/version.go	31.81% <0.00%> (ø)
cmd/app/grpc.go	48.27% <0.00%> (ø)
cmd/app/serve.go	19.33% <0.00%> (ø)
cmd/app/root.go	44.44% <0.00%> (ø)
pkg/ca/fileca/watch.go	100.00% <0.00%> (+50.00%)	⬆️

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