Support listing provider states

[mefellows]

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.

[mefellows]

Relates to #734

[github-actions]

🤖 Great news! We've labeled this issue as smartbear-supported and created a tracking ticket in PactFlow's Jira (PACT-3139). We'll keep work public and post updates here. Meanwhile, feel free to check out our docs. Thanks for your patience!