Add GLOBAL to conan targets so they can be used with ALIAS in CMake

[czoido]

Changelog: Feature: Set conan generated CMake targets as GLOBAL so that they can be used with an ALIAS for consumers.
Docs: conan-io/docs#1534

Making an ALIAS of an IMPORTED target is possible in CMake for versions >v3.10.3 (>v3.14 in case they are UNKNOWN too) but they have to be set as GLOBAL.
This PR sets those targets defined by Conan as GLOBAL so they can take advantage of this for consumers with modern CMake versions.

fixes: #3482

#tags: slow

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^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}

[memsharded]

Please add a couple of lines to the docs, update the link and merge.