Generate unique items for arrays with the "uniqueItems" property

[kevindew]

This re-calculates the values of an array if the "uniqueItems" property
is set and a non-unique item has been generated. This has been to help
resolve flaky tests that expect a random array and occasionally fail due
to the generation of an array with collisions.

As there is a risk that this can produce an eternal loop I've put in a
check that if there are very number of attempts to generate the unique
value then it should abort. This does produce a risk of failure if there
is a low amount of variance in the allowed variables.

This is to resolve issues such as:

GovukSchemas::InvalidContentGenerated: An invalid content item was generated.

This probably means there's a bug in the generator that causes it to output
invalid values. Below you'll find the generated payload, the validation errors
and the schema that was used.

...

Validation errors:
--------------------------

[
  {
    "schema": "869a7d30-f94e-5044-84e7-dc3c01c8f1ba#",
    "fragment": "#/details/featured_attachments",
    "message": "The property '#/details/featured_attachments' contained duplicated array values in schema 869a7d30-f94e-5044-84e7-dc3c01c8f1ba#",
    "failed_attribute": "UniqueItems"
  },
  {
    "schema": "869a7d30-f94e-5044-84e7-dc3c01c8f1ba",
    "fragment": "#/details/featured_attachments",
    "message": "The property '#/details/featured_attachments' contained duplicated array values in schema 869a7d30-f94e-5044-84e7-dc3c01c8f1ba",
    "failed_attribute": "UniqueItems"
  }
]

[gclssvglx]

This is looking great - thanks. Just a quick question...

[gclssvglx]

Great stuff - thanks @kevindew