Support for payload validation

[StarpTech]

We need the possibility to validate the incoming payload.

Popular valdiation librarys:

https://github.com/rjrodger/parambulator
https://github.com/hapijs/joi

We could use the same approach as in seneca.

//parambulator
.define(
      {
        a: 1,
        b: {required$: true}
      },
      function (msg, done) {
        done(null, {c: msg.b})
      })
//Joi
.define(
      {
        a: 1,
        b: Joi.required()
      },
      function (msg, done) {
        done(null, {c: msg.b})
      })

[davidmarkclements]

mu is bare essentials, I think this outside of mu core - especially because schema vs schemaless approaches - I'm happy to recommend Joi in the docs though

[mcollina]

IMHO we should support a definition time decorator mechanism. This would enable those behavior without the need of runtime support.

[pelger]

This could be implemented as a transport adapter ?