Issue #3346848: Add IP Pool selection feature

Don Allenrequested to merge
issue/sendgrid_integration-3346848:3346848-ip-pool-name into 8.x-2.x

Add ability to select an IP Pool for sending emails through SendGrid.

  • Add ip_pool_name configuration option
  • Fetch available IP Pools from SendGrid API
  • Display IP Pool dropdown in settings form (only when API key configured)
  • Apply selected IP Pool to outgoing emails
  • Add update hook for existing installations

Closes #3346848

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