Add pip_extra_index_url as a supported parameter in default builder

[thomasjpfan]

Why are the changes needed?

pip_extra_index_url was added in #2636, but the warning was not removed.

What changes were proposed in this pull request?

This PR adds pip_extra_index_url as a supported option.

How was this patch tested?

Ran image builder with no warning.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.87%. Comparing base (a8f68d7) to head (8e7e101).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2704      +/-   ##
==========================================
- Coverage   78.91%   78.87%   -0.05%     
==========================================
  Files         316      187     -129     
  Lines       24965    19232    -5733     
  Branches     4012     4017       +5     
==========================================
- Hits        19702    15169    -4533     
+ Misses       4548     3360    -1188     
+ Partials      715      703      -12     

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