Allow overriding the QNN SDK version via buck config

Summary:
`get_qnn_library_version()` was hardcoded to `2.51`
The value must name a vendored `fbsource//third-party/qualcomm/qnn/qnn-<version>` package.

Reviewed By: YIWENX14

Differential Revision: D126956903

Author: billmguo

shim_et/xplat/executorch/backends/qualcomm/qnn_version.bzl

@@ -1,2 +1,2 @@
 def get_qnn_library_version():
-    return "2.51"
+    return read_config("executorch", "qnn_version", "2.51")