Update on "[4/N] Add backend options map"

This is to manage the backend <-> BackendOptions map. Users will create the bakcend options map, and ET runtime will read the backend name, and dispatch the list of backend options to each backend.
exported-using-ghexport

Differential Revision: [D76149466](https://our.internmc.facebook.com/intern/diff/D76149466/)

Differential Revision: [D76149466](https://our.internmc.facebook.com/intern/diff/D76149466)

[ghstack-poisoned]

Author: cccclai

runtime/backend/backend_options.h

@@ -8,6 +8,7 @@
 
 #pragma once
 #include <executorch/runtime/core/error.h>
+#include <executorch/runtime/core/span.h>
 #include <cstddef>
 #include <cstring>
 #include <variant>
@@ -71,6 +72,15 @@ class BackendOptions {
     return Error::NotFound;
   }
 
+  executorch::runtime::Span<BackendOption> view() const {
+    return executorch::runtime::Span<BackendOption>(options_, size_);
+  }
+
+  // Non-const version that allows modification of the underlying data
+  executorch::runtime::Span<BackendOption> view() {
+    return executorch::runtime::Span<BackendOption>(options_, size_);
+  }
+
  private:
   BackendOption options_[MaxCapacity]{}; // Storage for backend options
   size_t size_; // Current number of options