DM-41345: Make dimension packer config optional

[parejkoj]

The default config should work in most cases, so it doesn't need to be required.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (facbe7f) 82.46% compared to head (63641dd) 82.43%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #381      +/-   ##
==========================================
- Coverage   82.46%   82.43%   -0.04%     
==========================================
  Files          92       92              
  Lines       10352    10354       +2     
  Branches     1951     1952       +1     
==========================================
- Hits         8537     8535       -2     
- Misses       1473     1475       +2     
- Partials      342      344       +2     

Files	Coverage Δ
...on/lsst/pipe/base/_observation_dimension_packer.py	92.15% <0.00%> (-3.77%)	⬇️

... and 1 file with indirect coverage changes

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