[WIP] Add min and max value for slash commands validation

[ankurp-bst]

This PR:

1. Adds MinValue and MaxValue fields in ArgDef struct.
2. Adds min and max value validations for slash commands.

[phenpessoa]

You can also do that in place if needed:

type foo struct {
	bar *float64
}

foobar := foo{func(in float64) *float64 { return &in }(10)}

But perhaps the generic approach is the best one indeed.

Maybe someday this will be a thing: https://github.com/golang/go/issues/45624