secretless can validate config files from CLI

[doodlesbykumbi]

Is your feature request related to a problem? Please describe. When there are issues with malformed configuration it's helpful to have a source of constructive feedback

Describe the solution you'd like A dry-run command that provides feedback in a standard and well documented format detailing missing, malformed etc. fields in the configuration, generally information useful to rectify whatever error there is.

$ secretlessctl validate -f ./secretless.yml

Describe alternatives you've considered ~A clear and concise description of any alternative solutions or features you've considered.~

Additional context ~Add any other context or screenshots about the feature request here.~

[sgnn7]

Related to #1063