samples: Bluetooth: GATT write commands with connection PHY Update

Update the Central and Peripheral GATT write command sample
to measure throughput across all supported connection PHYs.

Cover the sample in BabbleSIM CI testing to validate the 2M
PHY throughput after all supported PHYs have been changed
once.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>