Support runtime configuration

[GregoryComer]

🚀 The feature, motivation and pitch

We need a method for the user to pass in runtime configuration options, especially to specific delegates, but potentially to the core runtime as well. Motivating use cases include enabling/disabling workspace sharing or weight caching in the XNNPACK delegate. Ideally we would be able to expose both global and per-model configurations. @dbort has an older proposal for runtime configuration APIs that we can likely iterate on and implement.

Alternatives

No response

Additional context

No response

RFC (Optional)

No response

cc @JacobSzwejbka @dbort