[SUGGESTION] Switch from using application setting files to .NET user secrets

[seesharprun]

Today, many code samples have you entire your Azure Cosmos DB for NoSQL endpoint and credentials using an application settings file. This presents a few risks:

• This reinforces that saving your credentials in plain-text is a good idea (it's not)
• Customers could inadvertently check in real credentials to their forks

I recommend that we use .NET user secrets. The secrets are persisted on the user's machine and are not available in the source code as plain text credentials.

For each project you would need to:

1. The maintainers run dotnet user-secrets init on each original source project. It will add a non-sensitive GUID to the project. You just check in the project with the GUID so it "works" with .NET user secrets.
2. When it's time for the customers to add their endpoint, just tell them to run dotnet user-secrets set CosmosUri "<nosql-account-endpoint>"
3. For now, you can repeat the same step for keys. When they eventually migrate to Microsoft Entra authentication and RBAC, you can drop this additional step entirely
4. Instead of modifying the C# project file, they can just run the project as-is.

FYI @markjbrown