Add support for using .NET Aspire Dashboard (Standalone) as alternative observability stack

[ThorstenHans]

The .NET Aspire Dashboard could also act as single container observability stack. Compared to PLG and other stacks, standalone .NET Aspire Dashboard is way more lightweight.

After looking at the current implementation, I would prefer adding a single flag to spin otel setup. If the flag is present, a different compose.yaml file will be used. I would introduce a simple Stack type, to make this extensible (simplifies adding additional observability stacks in the future).

Other commands should not be affected from this change, as we will still leverage underlying docker compose.

[ThorstenHans]

.NET Aspire Dashboard supports header based authentication for its OTLP(s) endpoint. However, Spin does not support this yet.

I've created a corresponding issue at fermyon/spin#2880

Although we could run .NET Aspire Dashboard without using authentication for its OTLP endpoint(s), users will be presented with a prominent warning in the dashboard UI.