Provide attributes for supported runners, operating systems and runners in extensions

[pascalberger]

There's currently no structured way for an extension to express on which Cake runner, operating system or platform it runs. There might be issues with dependencies on different runners, issues with paths on different operating systems or an addin can target a more restrictive set of TFM than what Cake is available on. Current guidelines suggests to mention this in XML comment. It would be helpful to have attributes available:

• SupportedRunnersAttribute
• SupportedOperatingSystemsAttribute
• SupportedPlatformsAttribute

These addins can be used for two cases:

• On the website it allows to show this information as metadata for each addin. It also allows to filter to extensions which support a specific runner, operating system or platform.
• In Cake we can throw an error if user tries to run with an extension not supported in the current environment.

Existing addins which don't provide attributes should be treated as compatible, with a warning.

[augustoproiete]

A couple of thoughts to consider:

• Should some of these attributes specify "minimum version"? e.g. Windows 7, Windows 10, etc.
• Should Operating System define "bitness" (x86/x64)?