Define input variable types

This issue defines all input variable types and is a meta ticket for potential adjustments.

Right now we have:
* text
* password
* bool
* integer
* yaml

For reference:
https://github.com/elastic/package-registry/blob/33c9f9df5881a879ced083cc626ba973f928b32d/testdata/package/reference/1.0.0/dataset/reference/manifest.yml#L47
https://github.com/elastic/package-spec/blob/master/versions/1/dataset/manifest.spec.yml#L24

/cc @ruflin @ycombinator @jen-huang