Slot Swap Feature - Send signal to function extension on hostname/routing swap completes.

#### What problem would the feature you're requesting solve? Please describe.
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Azure Logic Apps is introducing zero downtime deployment by taking advantage of slot support in Azure Functions. Our workers depend on hostname and are initialized/registered during extension startup. During a swap, hostname/routing is updated after startup under the covers without recycling the app. The issue is once swap is complete, our workers reference the non-production hostname in a production slot.

At startup, we get the endpoint by calling `context.GetWebhookHandler()`, see code in Repo: [WorkflowExtensionProvider.cs](https://msazure.visualstudio.com/One/_git/AzureUX-BPM?path=/src/functionExtensions/Flow.WebJobs.Extensions/Startup/WorkflowExtensionProvider.cs&version=GBmaster&_a=contents)

`WorkflowExtensionProvider` initialize classes that are used to build URLs displayed in the portal but also in other components, such as webhook and HTTP triggers, callback URLs and inputs/outputs URLs

An example of the issue is that in the portal, the end-user will see the staging hostname in production slot. Below is one example.


#### Describe the solution you'd like
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The ideal solution is for Azure Logic Apps to subscribe or depend on a signal from host runtime when hostname/routing is updated during a swap. This ask will allow us to delay worker initialization/registration until hostname is updated. In addition, it would be helpful to know that the startup was triggered by a swap. This will help us to differentiate between, normal app recycle, and swap app recycle.

#### Describe alternatives you've considered
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We looked into synching the hostname after startup, however, there is no reliable path to perform the synch at the "right time". The solution/workaround implemented in [HostNameProvider.cs](https://github.com/Azure/azure-functions-host/blob/c64e6c4ec447fdc091426576ed60c4e8f2f0cd44/src/WebJobs.Script/HostNameProvider.cs) is not a viable solution/workaround for us.

#### Additional context
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Below is a screenshot of environment variables available at swap restart.


Start by reading HostNameProvider.cs in the referenced azure-functions-host revision and WorkflowExtensionProvider.cs in the AzureUX-BPM repository to understand current hostname resolution and worker initialization. Define the runtime signal and its swap-specific context, then verify that consumers can delay registration until hostname/routing updates complete without affecting normal app recycling.

Written by the indexing model from the issue text.