image_publisher: add field of view parameter (backport #985)

[mergify]

Currently, the default value for focal length when no camera info is provided defaults to 1.0 rendering whole approximate intrinsics and projection matrices useless. Based on this article, I propose a better approximation of the focal length based on the field of view of the camera.

For most of the use cases, users will either know the field of view of the camera the used, or they already calibrated it ahead of time.

If there is some documentation to fill. please let me know.

This PR should be straightforward to port it to Humble, Iron and Jazzy.

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This is an automatic backport of pull request #985 done by [Mergify](https://mergify.com).

[mergify]

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On branch mergify/bp/iron/pr-985
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Changes to be committed:
	modified:   image_publisher/include/image_publisher/image_publisher.hpp
	modified:   image_publisher/src/image_publisher.cpp

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   image_publisher/doc/components.rst

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