[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.

Implementation plan:

• [x] backend configuration
• [x] instances catalog in gpuhunt
• [ ] getting offers in dstack
• [ ] packer VM images
• [ ] running and terminating instances
• [ ] creating network resources
• [ ] creating dstack gateways
• [ ] client credentials without file paths