Centralize pipeline configurations

Use centralized templates and variables for pipeline configurations instead of duplicating or hardcoding values. This improves maintainability, ensures consistency, and makes platform-specific adaptations easier to manage.

Key practices:

1. Reference existing templates from /eng/pipelines/templates/ instead of creating duplicates
2. Include the image variables template /eng/pipelines/templates/variables/image.yml in pipelines that reference VM images
3. Use pool variables (LINUXPOOL, WINDOWSPOOL, MACPOOL) for pool names
4. Use demand-based configuration for Windows/Linux and vmImage pattern for Mac

Example:

# Good practice
variables:
  - template: /eng/pipelines/templates/variables/image.yml

jobs:
  - job: BuildAndTest
    pool:
      name: $(WINDOWSPOOL)
      demands: $(WindowsImageDemand)

Instead of:

# Avoid this approach
jobs:
  - job: BuildAndTest
    pool:
      name: azsdk-pool-mms-win-2022-general
      vmImage: windows-2022

This approach makes it easier to update image configurations across all pipelines and supports platform-specific configuration patterns.