Add structured `Mismatch` class

[JP-Ellis]

Have you read the Contributing Guidelines on issues?

• [X] I have read the Contributing Guidelines on issues.

Description

Replace the generic dict[str, Any] returned by a mismatch with a more informative class:

https://github.com/pact-foundation/pact-python/blob/5e52872b46fd6f959e958c130ebbe08ef8e11bbc/src/pact/v3/pact.py#L407-L412

Has this been requested on Canny?

No response

Motivation

A new class would facilitate the programmatic handling of mismatches, and avoid the use of Any.

Have you tried building it?

No response

Self-service

• [ ] I'd be willing to contribute this feature to Pact Python myself.