Adding candidate metadata key for tracking EP's OS driver version (#26616)

### Description
This change adds a well-known key name (`os_driver_version`)
corresponding to the OS driver version associated with an EP. We will
eventually flesh this out to enable retrieving it from the `OrtEpDevice`
if it's been populated, but for starters we reserve the name.

### Motivation and Context
We have a scenario in WebNN where the browser would like to get the
driver version associated with a given EP (this is to enable policy
against the driver, e.g. for maintaining a blocklist if a particular
driver version has a scenario-blocking bug in it). Having a mechanism to
retrieve the driver version via ORT would help with implementing this
feature.

---------

Co-authored-by: Aditya Rastogi <[email protected]>

Author: adrastogi

include/onnxruntime/core/session/onnxruntime_ep_device_ep_metadata_keys.h

@@ -9,6 +9,9 @@
 // Key for the execution provider version string. This should be available for all plugin EPs.
 static const char* const kOrtEpDevice_EpMetadataKey_Version = "version";
 
+// Key for the execution provider OS driver version.
+static const char* const kOrtEpDevice_EpMetadataKey_OSDriverVersion = "os_driver_version";
+
 // Prefix for execution provider compatibility information stored in model metadata.
 // Used when generating EP context models to store compatibility strings for each EP.
 // Full key format: "ep_compatibility_info.<EP_TYPE>"