Application Gateway support for new features

[amitsriva]

• Zone support for application-gateway create ([Network] AppGateway: Support for zones, autoscale config and trusted root certs #7789)
• Trusted Root Certificates (new child resource of appgw) ([Network] AppGateway: Support for zones, autoscale config and trusted root certs #7789)
• Autoscale Configuration (new property of appgw) ([Network] AppGateway: Support for zones, autoscale config and trusted root certs #7789)
• WAF configurability ([Network] AppGateway: Support for zones, autoscale config and trusted root certs #7789)
• WAF exclusion list ([Network] Implement AppGateway WAF exclusions #7920)
• Custom error pages ([Network] AppGateway: Support for zones, autoscale config and trusted root certs #7789)

[tjprescott]

Moving to sprint 50 for the remaining ask.