Make the Clean working dir available as a task

At the moment we have 2 ways of cleaning the working directory:

1. Force clean build: This option when enabled on the checkout task will clean the working directory before the next clone of the repo
2. Clean working dir after build. This will clean the whole working directory after each build.

Some users have a case where the build directory is not populated by a checkout task but rather by copying files via scripting and sometimes they will want to clear the working directory before moving some new files. The second option of using the clean working dir after each build does not really suit as that will mean the working directory will b empty after every build. It will be great to have a task ondemand to clean the working directory. We already have the clean working directory task by default in deployment projects. It will be great if this made available as a task that can be used at will.