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