[Feature]: Oracle Cloud Infrastructure backend

[jvstme]

Add support for Oracle Cloud Infrastructure. The main priority is on-demand GPU instances. Support for other types of instances may be added later.

Tasks

Essential, cannot release the feature without these:

• backend configuration
• instances catalog in gpuhunt
• getting offers in dstack
• packer templates
• running images from the marketplace
• running and terminating instances
• controlling disk size
• creating network resources
• creating compartments
• client credentials without file paths
• docs

Non-essential, may be added after the release:

• document restrictive permissions
• GitHub actions for publishing packer images
• multi-node tasks
• bare metal instances

May be implemented later:

• preemptible (spot) instances [Feature]: OCI spot instances #1375
• creating dstack gateways [Feature]: Running dstack-gateway on OCI #1376
• flexible instances - medium priority; flexible instances are CPU-only, but for newer OCI regions (e.g. Chicago) they are the only CPU VM offerings
• always free instances - lower priority; always free instances are very small CPU instances, but they could be useful for simple experiments

[r4victor]

@jvstme, should we close it as completed?