Add new Vespa integration with DocumentStore and Retrievers

[julian-risch]

Summary and motivation

To give Haystack users a simple option to use Haystack pipelines with Vespa for data storage and retrieval as an alternative to Haystack's other DocumentStore integrations, we should add an integration of Vespa and Haystack.

Detailed design

We should create a new integration based on pyvespa with a new VespaDocumentStore and Retrievers that use https://vespa-engine.github.io/pyvespa/query.html

Checklist

If the request is accepted, ensure the following checklist is complete before closing this issue.

Tasks

• [ ] The code is documented with docstrings and was merged in the main branch
• [ ] Docs are published at https://docs.haystack.deepset.ai/
• [ ] There is a Github workflow running the tests for the integration nightly and at every PR
• [ ] A new label named like integration:<your integration name> has been added to the list of labels for this repository
• [ ] The labeler.yml file has been updated
• [ ] The package has been released on PyPI
• [ ] An integration tile with a usage example has been added to https://github.com/deepset-ai/haystack-integrations
• [ ] The integration has been listed in the Inventory section of this repo README
• [ ] The feature was announced through social media