Change default function second argument to full helpers (from prefs)

[sekko27]

Support plan

• which support plan is this issue covered by? Community
• is this issue currently blocking your project? (yes/no): yes
• is this issue affecting a production system? (yes/no): no

Context

• node version: 10.13.0
• module version: 16.1.7
• environment (e.g. node, browser, native): node
• used with (e.g. hapi application, another framework, standalone, ...): standalone
• any other relevant information: -

How can we help?

Is it possible to pass the current state to the default value provider. It should be a help when using the pattern method (set default value based on the path for example)

const schema = Joi.object({})
  .pattern(
    /^.+$/, 
    Joi.object({
      type: Joi.string().default((ctx, options, state) => {
        // default value can be calculated based on state, for example env values
      })
    })
  );