Add a yaml schema file for the gitstream.cm file validation and error free IDE editing

[dorongutman]

Is your feature request related to a problem? Please describe.
I'm always frustrated when my IDE shows error on my gitstream.cm file, for example in the following line:

- {{ (pr.comments | filter(attr='content', term='/bypass_e2e') | filter(attr='commenter', list=['userA']) | some )}}

Describe the solution you'd like
I would like to have a yaml schema file I could reference in my gitstream.cm file so the IDE will show proper validation and maybe even auto complete of possible values.