Add label or legend param to date-input

[penx]

When I started using the date-input I was expecting to be able to pass in a label param.

{{ govukDateInput({
  id: "dob",
  namePrefix: "dob",
  label: "What is your date of birth?"
}) }}

In actual fact what I needed to pass was params.fieldset.legend.text:

{{ govukDateInput({
  id: "dob",
  namePrefix: "dob",
  fieldset: {
    legend: {
      text: "What is your date of birth?"
    }
  }
}) }}

The actual usage seems overly complex given that I expect most users just want to specify a string.

I accept the fieldset/legend isn't really a label in the html sense, so would propose one of the following:

1. allow a new label or legend prop to be passed as a string, that, if set, is used instead of fieldset.legend.text

govuk-frontend/src/components/date-input/template.njk

Line 90 in e70f59d

legend: params.fieldset.legend

becomes (written in JS/pseudocode as I am still learning nunjucks):

    legend: { text: params.label, ...params.fieldset.legend }

2. allow the fieldset param to optionally be passed in as a string

    legend: isString(params.fieldset) ? { text: params.fieldset } : params.fieldset.legend

This is low priority for me personally, but suggested in case others think it worthwhile.