Fix 'microxrcedds_agent' name and dependency (backport #279)

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Description

Before #237, the micro-ROS-Agent SuperBuild.cmake used to refer to the Micro-XRCE-DDS-Agent as xrceagent, which is inconsistent with the name exported by that package (microxrcedds_agent). In #237, the name and dependencies were updated, but not correctly (in line 24 the old name was kept), which caused some issues and was reverted on #239. The current approach works well for Vulcanexus, but can cause problems depending on the workspace configuration or build order.

This PR approaches this issue and tries to generate a consistent behaviour for building the package regardless of where the Micro-XRCE-DDS-Agent is coming from.

XRCE-DDS-Agent comes from	What ensures correct build order
this repo's SuperBuild.cmake clones/builds it	the DEPENDS ${_deps} (non-empty) at the end of SuperBuild.cmake
it is preinstalled in the system (Vulcanexus case)	nothing to order, it is already there
another package in your workspace	colcon, thanks to package.xml

Main changes

• The references to xrceagent are renamed as microxrcedds_agent
• The micro-ROS-Agent's package.xml now depends on microxrcedds_agent as well
• When microxrcedds_agent is not found, it is added as a dependency to the micro-ROS-Agent external project in SuperBuild.cmake
• microxrcedds_agent 3.0.2 or higher is needed, since that version starts exporting an EXACT dependency on spdlog 1.9.2, which ensures that the micro-ROS-Agent build doesn't choose a different version from somewhere else

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  This is an automatic backport of pull request Fix 'microxrcedds_agent' name and dependency #279 done by Mergify.

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