Create a standby machine for LSVD process with mounts

[wjordan]

Change Summary

What and Why:

This adds a special case to fly deploy for LSVD processes (detected by a FLY_LSVD_* environment variable defined in their machine config), to allow creating a standby machine for LSVD groups with mounted volumes.

An upcoming feature will allow an LSVD-enabled machine with a mounted volume to fail over its volume data to a standby machine, and this PR will allow automatically creating such a setup through fly deploy.

How:

• Add some logic to detect an LSVD process group, and use this to allow creating a standby machine even with a mounted volume configured
• Add a second mount for this configuration in provisionVolumesOnFirstDeploy

Related to:

#3800

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Documentation

• [ ] Fresh Produce
• [ ] In superfly/docs, or asked for help from docs team
• [ ] n/a

[wjordan]

One thing making me hesitate about this change is that as opposed to a created stopped Machine (which has no associated cost), creating a 'standby-volume' in advance adds extra ongoing cost to the standby setup that I'm not sure we want to introduce.