Add support to optional "select" fields

[nickfla1-mxm]

Currently the select field only support String, Number or Boolean types but in the case of optional fields forcing the user to select a value can be counter-intuitive.

Given the following example:

fields: {
  mySelect: {
    type: 'select',
    options: [
      { label: 'A', value: 'a' },
      { label: 'B', value: 'b' },
    ]
  }
}

if mySelect represents an optional value we could either

fields: {
  mySelect: {
    type: 'select',
    options: [
      { label: '', value: undefined }, // allow having `undefined` or `null` values
      { label: 'A', value: 'a' },
      { label: 'B', value: 'b' },
    ]
  }
}

or (cleaner in my opinion)

fields: {
  mySelect: {
    type: 'select',
    optional: true, // required: false alternatively
    options: [
      { label: 'A', value: 'a' },
      { label: 'B', value: 'b' },
    ]
  }
}

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I am available to contribute directly to this feature if approved!