[Spycloud] Create external import connector - phase 1

[Powlinett]

Proposed changes

• Use/adapt external import template
• Validate config/env variables
• Create Spycloud API client handling authentication, filters, retry strategy...
• Create Spycloud and OpenCTI classes
• Add unit tests

Related issues

• [SpyCloud] Create the connector #2563

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality using different use cases
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

I still need to write the README, but the code is ready for review 😇