Arm backend: Add MobileSAM Ethos-U example (#21397)

Add a MobileSAM prompt segmentation example for Ethos-U85-256.

Prepare the pinned official source in an external managed checkout.

Apply a small configurable-input patch instead of copying model code.

Use int8 globally and A16W8 for TinyViT attention.

Lower the complete graph to Ethos-U and compare output with FP32.

Emit delegation summaries, validation metrics, and debug artifacts.

Build a bare-metal Corstone-320 app using exporter metadata.

Validate inference on FVP through the Arm OOTB test.


cc @digantdesai @freddan80 @per @zingo @oscarandersson8218 @mansnils
@Sebastian-Larsson @robell @rascani

Signed-off-by: Usamah Zaheer <usamah.zaheer@arm.com>