feat: Provide Azure Front Door scraper

[tomkerkhove]

When implementing a new scraper; these tasks are completed:

• Implement configuration
• Implement validation
• Implement scraping
• Implement resource discovery
• Test end-to-end
• Document scraper
• Add entry to changelog

Metrics output:

# HELP promitor_demo_frontdoor_backend_health_per_backend Health percentage for a backed in Azure Front Door
# TYPE promitor_demo_frontdoor_backend_health_per_backend gauge
promitor_demo_frontdoor_backend_health_per_backend{resource_group="promitor-landscape",subscription_id="0f9d7fea-99e8-4768-8672-06a28514f77e",resource_uri="subscriptions/0f9d7fea-99e8-4768-8672-06a28514f77e/resourceGroups/promitor-landscape/providers/Microsoft.Network/frontdoors/promitor-landscape",instance_name="promitor-landscape",backend="promitor.io:443",app="promitor"} 100 1612809662196
# HELP promitor_demo_frontdoor_backend_health_per_backend_pool Health percentage for a backed in Azure Front Door
# TYPE promitor_demo_frontdoor_backend_health_per_backend_pool gauge
promitor_demo_frontdoor_backend_health_per_backend_pool{resource_group="promitor-landscape",subscription_id="0f9d7fea-99e8-4768-8672-06a28514f77e",resource_uri="subscriptions/0f9d7fea-99e8-4768-8672-06a28514f77e/resourceGroups/promitor-landscape/providers/Microsoft.Network/frontdoors/promitor-landscape",instance_name="promitor-landscape",backendpool="promitor-landscape_Docs",app="promitor"} 100 1612809662999

Discovery output:

[
  {
    "$type": "Promitor.Core.Contracts.ResourceTypes.FrontDoorResourceDefinition, Promitor.Core.Contracts",
    "Name": "promitor-landscape",
    "ResourceType": "FrontDoor",
    "SubscriptionId": "0f9d7fea-99e8-4768-8672-06a28514f77e",
    "ResourceGroupName": "promitor-landscape",
    "ResourceName": "promitor-landscape",
    "UniqueName": "promitor-landscape"
  }
]

Fixes #343