Support all VMSizeFlags in fly postgres create

[vboginskey]

Supersedes #4158

Change Summary

Before this change, fly postgres create only supported a set of bespoke VM sizes via the --vm-size flag, making it impossible to create a postgres cluster with, say, 1 shared CPU and 1 GB of memory.

As of this change, we create the underlying machine using the entire MachineGuest set of options, which allows for additional customization.

The legacy prompt and error behavior given no --vm-size or an invalid --vm-size is preserved.

Tested

• No VM options specified, selected preset configuration from prompt
• No VM options specified, selected custom configuration from prompt
• Specified --vm-size only
• Specified --vm-size and --vm-memory
• Specified --vm-memory only
• Specified an invalid --vm-size

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Documentation

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

[dangra]

Good job! thanks

I was going to provide strong feedback about how CreateCluster shouldn't access flags directly but accept everything through its params argument when I realized that is already happening. So your changes are good and at some point we will have to move that stuff out to the caller.