VS: Add support to indicate UTF-8 custom command pipe output encoding

Adds a flag to indicate that pipe output from a custom command should be interpreted as UTF-8 encoded. This change does not introduce a public way to set the flag, but generators that create internally-generated commands know if they are calling cmake, which uses UTF-8 pipes.

MSBuild added support for interpreting output of PreBuildEvent, PreLinkEvent, PostBuildEvent, and CustomBuildStep as UTF-8. This change will appear in Visual Studio 16.6 Preview 3. It is opt-in, and you need to add the StdOutEncoding tag. MSBuild treats these as property bags so if we emit the tag for earlier versions of Visual Studio it would be safely ignored. This change emits the StdOutEncoding tag and sets it to UTF-8 whenever the custom command UTF-8 pipe flag is set. This fixes globalization issues when the output from cmake contained characters that required MSBuild to interpret as UTF-8 before displaying them.