Support provider test lifecycle hooks

[mefellows]

It is common in testing frameworks to be able to hook into the lifecycle of tests to modify the state of the system in a fine-grained way. Current Pact SDK clients can support this on the consumer side fairly easily (as each test needs to be separated authored and has its own set of FFI lifecycle functions), however on the provider side things are essentially a single process - all tests run within the pactffi_verifier_execute function.

This presents a challenge for clients wanting to provide a native testing experience around the verification because there is no straightforward way to hook into the lifecycle.

JS/Go have proxy layers which were introduced for other reasons, but have been handy in enabling such behaviour:

Examples:

• JS: https://github.com/pact-foundation/pact-js/blob/master/docs/provider.md#before-and-after-hooks
• Golang: https://github.com/pact-foundation/pact-go/blob/master/docs/provider.md#before-and-after-hooks

This does create additional client complexity, however, and may not be workable for all languages.

Related issues:

• fix: allow hooks to run only once per interaction pact-js#1243

Potential approaches

1. HTTP "hooks" that fire, consistent with the current approach for state setup/teardown
   1. Support for a before hook would run before any state setup calls are made, and an after hook after all teardown calls were made
2. FFI callback functions

Another (potential) benefit of the hooks approach, would be to allow SDKs to also create "on-demand" test cases in the target frameworks. As it stands, most clients currently would run all provider tests within a single "test" case. Creating hooks that mark the start end of each test case would allow clients to dynamically create those tests.

I think this is probably best handled in another way, however (e.g. perhaps sending a structured JSON object back at the end of the verification, along with any related errors for each scenario, for the client to parse).