Feat/aggregate provider states by consumers

[YOU54F]

feat: group provider states by consumers

    {
      "providerStates" => [
        { "consumers" => ["Foo", "Foo2", "Foo3", "Foo4", "Foo5"], "name" => "product details", "params" => { "product_id" => "058925f7-1763-4dd9-a057-50ee265e33a0" } },
        { "consumers" => ["Waffle", "Waffle2"], "name" => "product list" },
        { "consumers" => ["Foo6"], "name" => "some other product list" }
      ]
    }

Fixes #789

[YOU54F]

Should this logic live elsewhere, rather than the decorator?

Would it be better to have this performed as part of a db query.

Just thinking of performance reasons, with a large set of consumer pacts, for a given provider.

[mefellows]

I'm not entirely sure of previous patterns across the code base, but pagination might be one way to address that performance concern.

[YOU54F]

Should we consider returning more information here.

Would it be useful for example, for the returned list in the api to contain the links to the latest pacts on the consumers main branch, which contain the provider states returned?

[mefellows]

A link might be a nice touch. Given it's just a template and doesn't require additional DB lookups it should be pretty speedy. But equally fairly straightforward to re-lookup using the HAL links so I think we could consider adding it later if people really need.