feat: basic template validation function

[buchdag]

This PR introduce three template functions intended to be used as validators.

All three take a string as last input and either return it unmodified or return an error, so that they can be used in a go template pipeline and prevent docker-gen from rendering the template if their respective validation fail.

• mustBeOneOf $slice $value: Validates that $value is one of the allowed string values in $slice.
• mustBeInt $value: Validates that $value is a base-10 integer string.
• mustBeIntInRange $min $max $value: Validates that $value is a base-10 integer string in the inclusive range $min..$max.

Example use:

{{ $value := somePipeline }}
{{ $allowedValues := list "foo" "bar" "baz" }}
{{ $sanitizedValue := $value | trim | mustBeOneOf $allowedValues }}
{{ /* the template rendering will fail at the above line if ($value | trim) does not evaluate as "foo", "bar" or "baz" /* }}