Add control plane identifier in DiscoveryServer responses

[Ferdudas97]

trafficstars

The purpose of this task was to increase observability. With these changes, we are able to tell from which control-plane envoy gets responses.

[codecov-commenter]

Codecov Report

Merging #245 (9860983) into main (90a3ccd) will increase coverage by 0.09%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main     #245      +/-   ##
============================================
+ Coverage     86.24%   86.33%   +0.09%     
  Complexity      351      351              
============================================
  Files            41       41              
  Lines          1141     1149       +8     
  Branches         93       93              
============================================
+ Hits            984      992       +8     
  Misses          112      112              
  Partials         45       45              

Impacted Files	Coverage Δ
...nvoyproxy/controlplane/server/DiscoveryServer.java	97.05% <100.00%> (+0.18%)	:arrow_up:
...oyproxy/controlplane/server/V3DiscoveryServer.java	94.11% <100.00%> (+0.56%)	:arrow_up:

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[liangyuanpeng]

kindly ping @rulex123 @nezdolik