[API] Progress endpoints

[iansk]

Hide all /api/v1/*/progress endpoints (including api/v1/images/progress). They've been deprecated.

[iansk]

Document the current versioned progress endpoints:

• [ ] /api/v21.08/cloud/discovery/progress
• [ ] /api/v21.08/images/progress
• [ ] /api/v21.08/serverless/progress

Question to answer in the docs: how should customers use these endpoints?

[iansk]

Given that are supported for the long term, let's get some decent documentation about how they should be used.

These endpoints supposedly report the progress of scan.

Tasks:

• [ ] Play with these endpoints in your own lab. Assess what they return
• [ ] Discuss with Ian know what you've learned about these endpoints
• [ ] After we've done our own research, we can ping engineering for more info (if required)