Add GitHub integration with agent_prompts and github_components

[julian-risch]

Summary and motivation

Related to deepset-ai/haystack-experimental#250
@sjrl brought up that one way to keep example files from the experimental Agent around is an integration deepset-ai/haystack-experimental#263 (comment)

Detailed design

Move github_components from experimental to a new integration
Move agent_prompts from experimental to a new integration
The idea is to enable users to run the example notebook (or a version with updated imports) after having installed this new integration

Draft PR: #1637

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 has been added to https://github.com/deepset-ai/haystack-integrations
• The integration has been listed in the Inventory section of this repo README
• There is an example available to demonstrate the feature
• The feature was announced through social media