Add HaystackEvaluator

[tholor]

We are adding external integrations to enable eval with uptrain, deepeval and RAGAs.
While those come with many nice out-of-the box capabilities, we also see some limitations with them that motivates us to create an own Evaluator, natively in Haystack:

1. Dependencies: deepeval and RAGAs come with several extra dependencies. We'd like to keep environments clean and small.
2. Choose a different model: Most default to using GPT-4. We want to allow users to pick different providers. Especially for privacy relevant use cases, where only open source models are acceptable.
3. Customization of model based metrics: Slightly lower prio than the other two but still important: users should be able to customize the prompt behind a metric (e.g. because slightly different wording is needed when they change an LLM or they want to provide different few-shot examples ...)

[masci]

Closing in favor of #7022