[WIP] Expose launch substitutions to frontends

[emersonknapp]

Basic Info

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Ticket(s) this addresses
Primary OS tested on	Ubuntu
Robotic platform tested on	N/A
Does this PR contain AI generated software?	No
Was this PR description generated by AI software?	No

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Description of contribution in a few bullet points

• Expose the launch Substitutions in nav2_common to the launch frontend, so they can also be used in YAML, XML, etc.
• Added @expose_substitution decorator
• Added parse class method
• Added setup.cfg to register the importlib entry point, so launch finds these
• Added content to __init__.py to force the substitutions to get registered on module import

Description of documentation updates required from your changes

Will need to add YAML/XML examples of nav2 launchfiles.

Description of how this change was tested

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Future work that may be required in bullet points

For Maintainers:

• Check that any new parameters added are updated in docs.nav2.org
• Check that any significant change is added to the migration guide
• Check that any new features OR changes to existing behaviors are reflected in the tuning guide
• Check that any new functions have Doxygen added
• Check that any new features have test coverage
• Check that any new plugins is added to the plugins page
• If BT Node, Additionally: add to BT's XML index of nodes for groot, BT package's readme table, and BT library lists
• Should this be backported to current distributions? If so, tag with backport-*.

[SteveMacenski]

LMK when you want me to review