demo/tcp: adds simple tcp client server demo

[shashankram]

Description:
This change adds simple tcp client and server apps to
demo tcp support in OSM. The client opens a connection,
sends a number of messages, and prints the server response.
The server simply echoes back the client's message.

Part of #1521, to be used as a simple demo to showcase TCP
traffic when OSM supports TCP. TCP support is still in progress
but the apps don't depend on it.

Affected area:

• New Functionality [ ]
• Documentation [ ]
• Install [ ]
• Control Plane [ ]
• CLI Tool [ ]
• Certificate Management [ ]
• Networking [ ]
• Metrics [ ]
• SMI Policy [ ]
• Security [ ]
• Tests [ ]
• CI System [ ]
• Performance [ ]
• Other [X]

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 #2161 (07ff50a) into main (dff9975) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2161      +/-   ##
==========================================
+ Coverage   58.32%   58.33%   +0.01%     
==========================================
  Files         145      145              
  Lines        6140     6140              
==========================================
+ Hits         3581     3582       +1     
+ Misses       2556     2555       -1     
  Partials        3        3              

Impacted Files	Coverage Δ
pkg/envoy/route/config.go	96.03% <0.00%> (+0.79%)	⬆️

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