[CostmapTopicCollisionChecker] Alternative constructor with footprint string

[doisyg]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses
Primary OS tested on	Ubuntu
Robotic platform tested on	Dexory Robot
Does this PR contain AI generated software?

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Description of contribution in a few bullet points

I needed to use the CostmapTopicCollisionChecker but with a custom footprint, NOT one served from a topic with FootprintSubscriber. So I added a constructor building a footpring from a string (same format as everywhere else).

Description of documentation updates required from your changes

Don't see any.

Description of how this change was tested

Dexory robot for a few days.

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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