aspire config set --secret?

There are places in Aspire where we read configuration values out of `IConfiguration` but generally speaking we expect the source for this information to be user secrets. A good example is when someone uses `AddConnectionString(...)`.

Unfortunately the Aspire CLI doesn't really give you the ability manipulate user secrets. You could set a configuration values for a connection string but it would go into .aspire/settings.json.

I'm thinking that we should probably add first class secrets support to the `aspire config` command.

I think the way this would work is that if you specify `--secret` on the command-line instead of updating the config file, it would invoke `dotnet user-secrets` instead (using the app host project as context).

I don't know how this would work with polyglot app hosts though since the user secrets ID is embedded within the project file. Maybe we need to hoist that into `.aspire/settings.json` and pass it along when running the app host?