Add support for mimic tags in URDFs

[apockill]

Some URDF's have a tag called mimic, which indicates that a certain joint should always copy another joints value. This PR adds support for namespacing those cases.

Prior to this, joint_state_publisher would crash upon loading a urdf that has this feature.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 15 lines in your changes missing coverage. Please review.

Project coverage is 90.09%. Comparing base (227d504) to head (bc2fabd).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...de_helpers/node_helpers/nodes/node_helpers_node.py	48.27%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
- Coverage   90.29%   90.09%   -0.21%     
==========================================
  Files         174      176       +2     
  Lines        6370     6413      +43     
  Branches      775      778       +3     
==========================================
+ Hits         5752     5778      +26     
- Misses        520      536      +16     
- Partials       98       99       +1     

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