[Core] Disk tier best is not the best possible disk

[romilbhardwaj]

On AWS, sky launch -c aws --cloud aws --disk-tier best provisions a gp3 EBS with 7000 IOPS and 437MB/s throughput.

On the other hand, the "best" possible EBS (discounting io2) is io1 with 64000 IOPS and 1 GB/s throughput:

Similar behavior may exist on other clouds, need to verify.

I understand SkyPilot is using its "high" disk tier as the "best", but that is ~9x fewer IOPS and half the throughput you can get from the "best" EBS. Using the best possible configuration is particularly important for performance of disk-heavy workloads.