Add SagemakerGenerator

[mathislucka]

Is your feature request related to a problem? Please describe.
Sagemaker is widely used for LLM inference in production use cases. We should support Sagemaker in 2.0.

Describe the solution you'd like
Generator similar to the Sagemaker support in 1.x

Tasks

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1. Implementation: Implement SagemakerGenerator #275
2. Docs are published at https://docs.haystack.deepset.ai/ Documentation for SagemakerGenerator #296
3. There is a Github workflow running the tests for the integration nightly and at every PR
4. A label named like integration:<your integration name> has been added to this repo
5. The labeler.yml file has been updated
6. The package has been released on PyPI
7. An integration tile has been added to https://github.com/deepset-ai/haystack-integrations: Integration card for Sagemaker integration #292
8. The integration has been listed in the Inventory section of this repo README: Add Sagemaker integration to the inventory table in the README #290
9. There is an example available to demonstrate the feature: https://colab.research.google.com/drive/1S3QO4OWx3oK58XVAVo0SissekA6NViaC#scrollTo=EX5oCws-etEH
10. The feature was announced through social media