Support Azure App Configuration service as special configuration source like Key Vault.

#### What problem would the feature you're requesting solve? Please describe.
Today we can load values into App Service / Function configuration from Key Vault by using the `@Microsoft.KeyVault(SecretUri=https://myvault.vault.azure.net/secrets/mysecret/)` format.

There is no similar way to achieve the same from Azure App Config which can lead to issues when a scale-out event causes a Function to make multiple calls to the App Configuration API and be throttled.

While there are ways to manage this with .NET (https://docs.microsoft.com/en-us/azure/azure-app-configuration/quickstart-azure-functions-csharp) there is no consistent, language-agnostic way to do this for all supported languages (or BYO languages developed via custom handlers).

#### Describe the solution you'd like
Provide a new configuration source definition in the form of `@Microsoft.AppConfiguration(ConnectionUri=https://connection/string;ConfigKey=Your:Config:Item)` which will load a specified App Configuration item into memory and make it available to any running application as though the config item were stored in the local App Config settings.

#### Describe alternatives you've considered
I've looked at the existing per-language options and found they don't offer support for languages such as Python or Node. So there haven't really be any alternatives considered.

#### Additional context
None.

The issue proposes a new @Microsoft.AppConfiguration source for Azure Functions, using ConnectionUri and ConfigKey, to load an App Configuration item into local settings. Start by locating the existing Key Vault configuration-source implementation in the host; done means the source works consistently across supported and custom-handler languages without per-language setup.

Written by the indexing model from the issue text.