Expose StringJoinSubstitution to frontend

[baconbrot]

This PR exposes StringJoinSubstitution from #843 to the frontend, as suggested by @christophebedard in #843.

Usage

(i.e. with delimiter=. and string components=[https://$(var subdomain), ros, org]):

%YAML 1.2
---
launch:
  - arg:
      name: subdomain
      default: 'wiki'
  - log:
      message: "$(string-join . https://$(var subdomain) ros org)"

results in terminal output

[INFO] [launch.user]: https://wiki.ros.org

[christophebedard]

Other than some minor comments, this looks good.

Would you mind adding tests for frontends, i.e., YAML and XML? See the tests I added to launch_xml and launch_yaml in #802. Also, it would be good to add XML and YAML examples to the Python class docstring.

[christophebedard]

Thanks for adding the tests!

Other than some minor suggestions, this looks great!

[christophebedard]

Just a thought, but given that we have a LaunchDescription here, I think it would be more appropriate to call ld.entities[n]. Same below and for the other tests.

[christophebedard]

Type annotation in launch has progressed a lot lately. Could you rebase on rolling and annotate the return type? I think this should do it:

Suggested change

	def parse(cls, data: Sequence[SomeSubstitutionsType]):
	def parse(
	cls, data: Sequence[SomeSubstitutionsType],
	) -> Tuple[Type['StringJoinSubstitution'], Dict[str, Any]]:

[christophebedard]

After the change to the return type annotation, can you try to remove the type annotation for kwargs here and see if mypy is fine with it? Otherwise, you can try to just directly return a dict:

Suggested change

	kwargs: Dict[str, SomeSubstitutionsType | Iterable[SomeSubstitutionsType]] = {}
	kwargs['delimiter'] = data[0]
	kwargs['substitutions'] = data[1:]
	return cls, kwargs
	return cls, {'delimiter': data[0], 'substitutions': data[1:]}