[Command validation] allow custom logic for IsZero

[hansbogert]

Is your feature request related to a problem? Please describe.
Currently we have 2 cases, which the CheckCommand does not handle correctly imo.
We can't have the following commands with the following values:

OnlyValidStructIfAllFieldsAreNonZero {
  bar: ""
  bar2: "asdf"
}

Foo {
  bar: 0
}

• OnlyValidStructIfAllFieldsAreNonZero is allowed by CheckCommand as at least one field is non-zero (bar2)
• Foo is allowed by CheckCommand as (ironically) the number 0 is considered non-zero.

Describe the solution you'd like
If we introduce a IsZeroer interface, types used in Commands can optionally implement IsZero() and thus have their own semantic meaning of being zero-valued or not.

Describe alternatives you've considered
No alternatives