Support literal/union types in pipe parameter validators

[gnzjgo]

Description

Currently pipe parameter validators (p.string(), etc.) don't support constraining values to a set of allowed literals (union/literal types).

This would be useful for parameters like sort order or filter modes where only specific values are valid.

Example

params: {
  order: p.string().literals('asc', 'desc'),  // type: 'asc' | 'desc'
  status: p.string().literals('active', 'inactive', 'pending'),
}

This would provide both TypeScript type safety and runtime validation.