feat: add data sources for listing GitHub App installations in an organization

[atilsensalduz]

This PR introduces a new data source, github_app_installations, to enable listing all installed GitHub Apps within an organization. This addition enhances integration with existing resources such as github_app_installation_repository, providing better automation and management capabilities for GitHub App permissions.

What's New?
New Data Source: github_app_installations
Allows fetching all installed GitHub Apps in an organization:

data "github_organization_app_installations" "all_apps" {}

Example Use Case:
This data source can be integrated with the github_app_installation_repository resource to manage app permissions on specific repositories:

# Local value to find the correct app installation by slug
locals {
  # Find the index of the desired app by its slug
  app_index = index(
    data.github_organization_app_installations.all_apps.installations[*].slug,
    "desired-app-name"  # Replace with your actual app slug
  )
  
  # Get the app_id using the found index
  app_installation_id = data.github_organization_app_installations.all_apps.installations[local.app_index].id
}

# Link the repository to the app installation
resource "github_app_installation_repository" "some_app_repo" {
  installation_id = local.app_installation_id
  repository     = github_repository.some_repo.name
}

API Reference: https://docs.github.com/en/rest/orgs/orgs?apiVersion=2022-11-28#list-app-installations-for-an-organization

Related Issue: #2570

[github-actions]

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[deiga]

Hey 👋

Thank you for your contribution!

I've requested a few changes to make this fit into the provider even better :)

[deiga]

@atilsensalduz Please rebase and go through the comments

[atilsensalduz]

@atilsensalduz Please rebase and go through the comments

Hey @deiga , apologies for the late reply. I somehow missed this PR. Thanks so much for the review! I’ve worked through your suggestions.

[atilsensalduz]

Hi @deiga , I've changed the PR according to your suggestions. Could you please take a look again when you have time? Thanks in advance.

[stevehipwell]

@atilsensalduz could you please rebase this PR?

[stevehipwell]

Thanks for the PR @atilsensalduz, I've added a couple of comments. Could you also add support for single_file_paths, created_at & updated_at.

[stevehipwell]

@atilsensalduz this is looking good. Have you run the test locally, if so could you please attach a screenshot of the output?