ADR: Supporting local development alongside use of Azure AppConfiguration

Given the desire to utilise Azure AppConfiguration to simplify configuration management, we need to ensure that this online dependency does not impair the inner dev-loop.

The azure-cli supports exporting configuration from an AppConfiguration store to a local, unencrypted file. This file could then be used to satisfy configuration requirements when the AppConfiguration store is unavailable.

It is expected that the handling of this AppConfiguration support and offline fallback would be handled as part of the tooling offered by this library.

This ADR aims to compare the options for such an approach and recommend a way forward. Previous discussions have resulted in the following suggestions, though other may present themselves:

1. Exported to a file in the source repo relying on `.gitignore` to avoid being committed - (any secrets would be stored in cleartext in the source repo)
1. Exported to a file that is processed via the [`dotnet user-secrets` tooling](https://docs.microsoft.com/en-us/aspnet/core/security/app-secrets?view=aspnetcore-6.0) to store them outside the source repo - (any secrets would be stored in cleartext in user's profile area)
1. Either of the above options could be extended to use [DPAPI](https://docs.microsoft.com/en-us/dotnet/standard/security/how-to-use-data-protection) to avoid storing any secrets in cleartext