Add hvPlotAgent as default

[ahuang11]

Initially I thought there was a bug with initializing a list of agents #739

Then, I was surprised to find that it was not in the default agents given that Lumen is a data viz library (I think?) and depends on hvplot https://github.com/holoviz/lumen/blob/main/pyproject.toml#L31

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.22%. Comparing base (049a997) to head (b57f357).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #740   +/-   ##
=======================================
  Coverage   61.22%   61.22%           
=======================================
  Files         101      101           
  Lines       12352    12352           
=======================================
  Hits         7562     7562           
  Misses       4790     4790           

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[philippjfr]

The thing here is that both hvPlot and Vega are good choices and in my testing I've found the vega agent more useful.