pkg/proxy: make/simplify interface more extensible

[gyuho]

Extend proxy for more advanced corrupt and packet drop testing.

For #5614 and #9563.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@03cf9c4). Click here to learn what that means.
The diff coverage is 59.79%.

@@            Coverage Diff            @@
##             master    #9571   +/-   ##
=========================================
  Coverage          ?   72.22%           
=========================================
  Files             ?      368           
  Lines             ?    31357           
  Branches          ?        0           
=========================================
  Hits              ?    22648           
  Misses            ?     7081           
  Partials          ?     1628

Impacted Files	Coverage Δ
pkg/proxy/server.go	59.58% <59.79%> (ø)

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