Use spot instances to reduce transfer costs

[parasj]

Fixes #511

Problem: Skyplane VMs tend to be left running for extended periods in the case where there is an exception in the CLI. Although egress fees dominate the cost of transfers, VM costs remain significant. To improve cost-effectiveness of Skyplane, we introduce a new option to control scheduling on spot instances.

Implementation stages:

• (done) Provisioning spot VMs: in {aws, azure, gcp}_cloud_provider.py, we expose a use_spot_instances flag
• (done) Exposing spot instances in the CLI: we add additional configuration options to enable spot instances
• (todo) Document new flags in the documentation under a "cost optimization" section
• (todo) NEEDS DESIGN, recovering when spot VMs die: TODO