Feature: add definitions to MSBuild (and build_sln_command)

[SSE4]

closes #4073

Changelog: Feature: add definitions to MSBuild build helper (and tools.build_sln_command())
Docs: conan-io/docs#1024

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
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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.}

/cc @solvingj

[solvingj]

Amazing, looks really good and clean to me.

[SSE4]

I gonna to update this to use MSBuild instead of build_sln_command in tests

[SSE4]

updated tests to use MSBuild

[memsharded]

Please fix conflicts when possible. Thanks!

[SSE4]

fixed

[lasote]

Missing docs? Missing "Doc: XXX" tag

[SSE4]

added docs