Add validate command

[tetienne]

I would like to add a bk validate <path/to/file.yml> to the various git hooks we uses at my enterprise.
The purpose of this command would be to check/validate the syntax of the given file is well a correct pipeline and the syntax is ok.
It would also allow to validate dynamic configuration: my_super_script.sh | bk validate -

[mcncl]

👋 Thanks for this issue. It seems like it would be a useful tool, however we don't have scope to work on something like this right now and I'm not sure how it would actually eventually look in the CLI. I like the idea of a pipeline validator, but it would likely just validate the YAML in a file, rather than support dynamic function etc right now.