Add recipe for libignition-gazebo4

[traversaro]

Related to #13551 . This adds the Ignition Gazebo simulator, the successor of the widely used Gazebo robotics simulator (http://gazebosim.org/). Ignition Gazebo is part of the ignition robotics libraries (see https://ignitionrobotics.org/).

Support for macOS and Windows is currently blocked by the following issues in dependent libraries:

• Fix macOS build libignition-sensors-feedstock#3
• Enable compilation of part that depend on sdformat and dartsim on Windows libignition-physics-feedstock#12

So for the time being only Linux is enabled, support for the other platforms will be added as soon as the dependencies are ready.

Checklist

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[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I wanted to let you know that I linted all conda-recipes in your PR (recipes/libignition-gazebo4) and found some lint.

Here's what I've got...

For recipes/libignition-gazebo4:

• Selectors are suggested to take a <two spaces>#<one space>[<expression>] form. See lines [16]

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipes/libignition-gazebo4) and found it was in an excellent condition.

[traversaro]

For @wolfv the approval in #14323 (comment) probably means he is ok with being a maintainer also of this recipe, for @Tobias-Fischer and @j-rivero an explicit go would be nice, thanks!

@conda-forge/help-c-cpp the recipe is ready for review.

[Tobias-Fischer]

Fine with me :)

[traversaro]

Temporary removed @j-rivero from mantainers to get this merged, @j-rivero feel free to open a issue or a PR to add you back as maintainer once the feedstock is created.