debugger: Expose all feature flags via /debug/feature-flags endpoint

[draychev]

This PR adds the /debug/feature-flags endpoint to the debug server, which shows current enabled feature flags.

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Please answer the following questions with yes/no.

• Does this change contain code from or inspired by another project? If so, did you notify the maintainers and provide attribution?
  no

[codecov-io]

Codecov Report

Merging #1926 into main will decrease coverage by 0.00%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main    #1926      +/-   ##
==========================================
- Coverage   58.50%   58.49%   -0.01%     
==========================================
  Files         130      131       +1     
  Lines        5256     5262       +6     
==========================================
+ Hits         3075     3078       +3     
- Misses       2178     2181       +3     
  Partials        3        3              

Impacted Files	Coverage Δ
pkg/debugger/feature_flags.go	20.00% <20.00%> (ø)
pkg/debugger/server.go	100.00% <100.00%> (ø)
pkg/envoy/route/config.go	95.83% <0.00%> (+0.83%)	⬆️

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