Support discovery of TinyXML2 by pkgconfig

[fedegiova]

Allow to discovery TinyXML2 by pkg-config is a -config.cmake file is not available.

Since tinyxml2 is built with meson in Yocto it's configuration gets deployed by pkg-config (because meson cannot generate CMake target files). In this case, the current FindTinyXML2.cmake will find out the path of tinyxml2.h and of tinyxml2.so by looking into the recipe-sysroot folder, which path in going to get expanded into the fastrtps-target.cmake generated when fastdds is built. The end result is that we're going to have a system-wide deployed config mode package that points to a directory that won't (probably) not exist anymore when other dependent targets are built.

Since it's not possible to force meson to produce a tinyxml2-target.cmake file, the most reasonable solution is trying to look for the library by pkg-config.

With this patch applied, the generated fastrtps-target.cmake will have a reference to the target PkgConfig::TinyXML2 instead of a full path.

Note that embedding TinyXML2 without symbol leaking it's not possible since they declare interfaces with default visibility. Which means that while creating an image we must have one and only one version of TinyXML2 around. Which basically makes impossible to use the bundled one in thirdparty

Contributor Checklist

• Commit messages follow the project guidelines.
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• ❌ Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
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• ❌ Any new configuration API has an equivalent XML API (with the corresponding XSD extension)
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• ❌ Changes are API compatible.
• ❌ New feature has been added to the versions.md file (if applicable).
• ❌ New feature has been documented/Current behavior is correctly described in the documentation.
• ❌ Applicable backports have been included in the description.

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