feat: Implement PipelinePair primitive and related utility functions

[shadeMe]

This is the first of a series of PRs that revolves around implementing an evaluation harness (c.f deepset-ai/haystack#7526).

PipelinePair is a utility class that allows for two pipelines to be used in tandem. The motivation behind this abstraction is to reduce the amount of glue code it takes to connect a, say, RAG pipeline with an evaluation pipeline. It aims to be generic enough work with arbitrary pipelines while making special accommodations to ensure that pipelines of different "shapes" can be connected without much extra work from the user. "Shape" here refers to the modality of pipeline inputs and outputs, e.g: a typical RAG pipeline takes a query and returns an answer whereas a typical RAG evaluation pipeline will take batches of queries and answers. Connecting two such pipelines is handled by automatically aggregating and deaggregating inputs and outputs at the interface between the pipelines.

Blocked by #2.

[davidsbatista]

This was a rather a big one, I tried to cover as much detail as possible, the logic looks OK to me, but I might have missed some edge cases - maybe checking what the test coverage shows to be sure we are testing every relevant piece of code.

[julian-risch]

I agree this idea will simplify running a pipeline with the outputs of another pipeline as inputs. It looks quite flexible. 👍 Let's try it out with some examples and get feedback from users.

[julian-risch]

Looking forward to seeing this in action in RAGEvaluationHarness!

[coveralls]

Pull Request Test Coverage Report for Build 9189096904

Details

• 251 of 251 (100.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+10.7%) to 99.615%

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Totals
Change from base Build 9186835496:	10.7%
Covered Lines:	259
Relevant Lines:	260

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💛 - Coveralls