Add a "duration" string format

[ptodev]

I've been trying to schematise the config of an application which uses time.Duration. Ideally, I'd like the generated Go code to use time.Duration.

This PR is still at a very early stage. Please feel free to let me know if you approve the overall direction in which the work is going. Any help would be much appreciated! 😊

[omissis]

hey @ptodev the idea looks promising, are you still interested in pushing this one further?

[ptodev]

Hi, @omissis! I'd be very happy to address any feedback you have.
Thank you so much for taking a look!

I believe the code mostly generates what it needs to, but it's still WIP due to the TODOs.

[omissis]

nothing against this one, so feel free to finish it and set it to "Ready for review" once all the todos and required fixes are done! thank you! 💪

[ptodev]

@omissis Do you think it is ok to leave these panic statements here for now? It looks like the current code doesn't really have a way to guard aganst such invalid defaults, and so I'm not sure how to propagate the errors. For example, this schema:

    "myNumber": {
      "type": "number",
      "default": "asdf"
    },

... produces code like this:

type Primitives struct {
  ...
  MyNumber float64 `json:"myNumber,omitempty" yaml:"myNumber,omitempty" mapstructure:"myNumber,omitempty"`
  ...
}

func (j *Primitives) UnmarshalJSON(b []byte) error {
  ...
  if v, ok := raw["myNumber"]; !ok || v == nil {
    plain.MyNumber = "asdf"
  }
}