Support A3 Mega GCP clusters with GPUDirect-TCPXO

[r4victor]

Closes #2447

The PR adds support for GPUDirect-TCPXO on GCP A3 Mega instances:

• Adds extra_vpcs parameter to GCP config to configure VPC for GPU NICs.
• Supports placement groups for GCP – fleets with placement: cluster now have instances in the same collocated placement.
• Adds a Github Action to build a debian-based VM image with GPUDirect-TCPXO – this is now used for A3 Mega instances.
• Adds A3 Mega specific commands to run on cloud-init that configure GPUDirect-TCPXO.
• Adds an example on how to configure and use GCP A3 Mega with GPUDirect-TCPXO.

TODO:

• Currently to use GPUDirect-TCPXO optimally, certain env variables need to be set up. An example shows how it can be done. The next step may be to configure all the env vars GPUDirect-TCPXO for A3 Mega automatically. Although, it needs to be researched if hardcoding them is a good idea.
• Supporting GPUDirect-TCPX.