Allow values in a request to be changed by the provider state

[tucker-m]

This is something that we've been using internally for a few weeks, and I wanted to suggest it as feature in Pact. This has been useful for setting object IDs in URLs and authentication tokens in the headers; you might not be able to know those values until after the provider state has been set up. After searching through the various pact-foundation repositories, it looks like other people have run into similar needs.

You can create a "provider parameter" by doing:

Pact.provider_param("/users/:{user_id}/posts", user_id: '1')

When the test is run on the consumer side, the URL will be /users/1/posts.

On the provider side, in your provider state, you can do this:

user = User.create!
provider_param :user_id, user.id

Let's say that user.id wasn't 1, but that it actually got assigned 2. When you run rake pact:verify, the URL will be /users/2/posts.

This is also useful with authentication tokens or UUIDs, where they are randomly generated and you have no way of knowing a valid one without knowing something about the provider state beforehand.

[YOU54F]

Will be revisiting this as part of pact-foundation/pact-ruby#319

it relates to the ProviderState Generator in version 4 of the pact-spec

https://github.com/pact-foundation/pact-specification/tree/version-4?tab=readme-ov-file#supported-generators

As shown in use in pact-js consumer facing DSL here

https://github.com/pact-foundation/pact-js/blob/master/docs/matching.md#provider-state-injected-values

Note: I need to test what happens in the rust core with the generator rule written in v2/v3 spec, as to what happens on the verification side.