docs: Add example for developing agents with custom VPP plugins

[ondrej-fabry]

This PR adds new code example and updates existing one for developing customized agents:

• New example: Custom VPP Plugin

  Example Custom VPP plugin contains a working example of custom agent which adds support for a custom VPP plugin. This example can serve as a skeleton code for developing custom agents adding new VPP functionality that is not part of official VPP Agent.

• Updated example: Custom API Model

[codecov]

Codecov Report

Merging #1665 into master will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1665      +/-   ##
==========================================
- Coverage   58.33%   58.25%   -0.08%     
==========================================
  Files         594      365     -229     
  Lines       47855    29883   -17972     
==========================================
- Hits        27914    17408   -10506     
+ Misses      17078    11043    -6035     
+ Partials     2863     1432    -1431     

Flag	Coverage Δ
#e2e	?
#unittests	58.25% <ø> (ø)

[sonarqubecloud]

SonarCloud Quality Gate failed.

0 Bugs
10 Vulnerabilities (and 0 Security Hotspots to review)
14 Code Smells

No Coverage information
69.5% Duplication