Create iterator for custom height field in pointcloud.

[greganderson-vermeer]

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Basic Info

Info	Please fill out this column
Ticket(s) this addresses	#5585
Primary OS tested on	Ubuntu
Robotic platform tested on	Vermeer Balehawk
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

• Added iterator in for a custom height field in our point cloud iterator

Description of documentation updates required from your changes

• If completed in a configurable method it would require new parameters, so would need to add that to default configs and documentation page
• I added some capabilities, need to document them

Description of how this change was tested

• No unit tests have been written or updated, but this has been tested on my physical robot platform in production for 6 weeks.

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Future work that may be required in bullet points

• I know in this form this is not going to be commonly useful and would need to be setup to be configurable for end users. (see issue description) I would be willing do the configuration setup work but guidance or direction to the best example of how this is typically handled in the nav2 Eco-system would be appreciated.

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]

See #5585 (comment)

Basically: Just have the height_field be a parameter with a string type default to "z". Then you can set this in your yaml to "height" and no other code needs to change (i.e. no new iterators or changes to the check, just have iter_z use that parameterized string value)