[Flashpoint]: Enhance Flashpoint connector

[romain-filigran]

Proposed changes

Rework of the Flashpoint connector, which solves issues but also improves the way we model and ingest alerts from Proofpoint.

Global

• Implement the standard scheduling and back-pressure mechanism. Previous "interval" configuration option stay maintained to not introduce breaking change.
• Generate a work_id per collection to ingest

Flashpoint Alerts

• Better Modeling of Alerts: Refine the structure used to model Flashpoint alerts in OpenCTI
• Use a more efficient API to retrieve alerts
• Consideration of Data Exposure Alerts

Flashpoint Indicators (MISP Feed)

• No longer import MITRE attack pattern identifiers as labels
• Align indicator ingestion scheduling

Related issues

• [Flashpoint]: Enhance Flashpoint connector #3279
• [Flashpoint] interval not taken into account #3096

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