msbuild generator custom configs

[memsharded]

Changelog: Feature: Allow (experimental) custom configuration of the msbuild generator.
Changelog: Feature: Rename msbuild generator to MSBuildDeps and use the new generate() method.
Docs: conan-io/docs#1952

Simplified from https://github.com/conan-io/conan/pull/7754/files

This is ONLY the msbuild part. There are 2 external things to be addressed too, but lets discuss those in other issues:

• The import structure for tools, helpers, etc.
• The config mechanism to propagate things from config and command line to the recipes and codebase.

#tags: slow

[jgsogo]

Some changes/suggestiongs, and main comment: IMO, we should remove the config_filename, it adds a degree of freedom that is not needed and it can be misleading because it is not considered in the condition:

def toolchain(self):
   ms = MSBuildDeps(self)
   ms.configuration = "My"+str(self.settings.build_type)
   ms.platform = "My"+str(self.settings.arch)
   ms.config_filename = 'optTrue' if self.options.opt1 else 'optFalse'
   ms.generate()

We get two different files, but we cannot reach one of them, which one? It depends on the install order...

[jgsogo]

Just a couple of comments