feat: aggregated provider states

[YOU54F]

Provide an endpoint to retrieve provider states to aid discoverability.

Initial work performed in https://github.com/pact-foundation/pact_broker/tree/feat/aggregated-provider-states by @bethesque

Allowed methods: GET

Path: /pacts/provider/{provider}/provider-states

This resource returns a aggregated de-duplicated list of all provider states for a given provider.

Provider states are collected from the latest pact on the main branch for any dependant consumers.

Example response

{
    "providerStates": [
        {
            "name": "an error occurs retrieving an alligator"
        },
        {
            "name": "there is an alligator named Mary"
        },
        {
            "name": "there is not an alligator named Mary"
        }
    ]
}