Mod: update the comments in filter directory

[williamfeng323]

What this PR does:
Added comments to filter directory

Which issue(s) this PR fixes:
Fixes #569

[codecov-commenter]

Codecov Report

Merging #586 into develop will increase coverage by 0.25%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #586      +/-   ##
===========================================
+ Coverage    66.43%   66.68%   +0.25%     
===========================================
  Files          184      185       +1     
  Lines         9712     9749      +37     
===========================================
+ Hits          6452     6501      +49     
+ Misses        2620     2611       -9     
+ Partials       640      637       -3     

Impacted Files	Coverage Δ
common/extension/auth.go	0.00% <ø> (ø)
filter/filter_impl/active_filter.go	66.66% <ø> (ø)
filter/filter_impl/auth/accesskey_storage.go	100.00% <ø> (ø)
filter/filter_impl/auth/consumer_sign.go	60.00% <ø> (ø)
filter/filter_impl/auth/default_authenticator.go	80.00% <ø> (ø)
filter/filter_impl/auth/provider_auth.go	52.94% <ø> (ø)
filter/filter_impl/auth/sign_util.go	69.23% <ø> (ø)
filter/filter_impl/echo_filter.go	85.71% <ø> (ø)
filter/filter_impl/execute_limit_filter.go	73.80% <ø> (ø)
filter/filter_impl/generic_filter.go	67.69% <ø> (ø)
... and 35 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4af26b9...e03e24e. Read the comment docs.