ROX-24169: client auth in OCM form

[stehessel]

Description

Add OCM service account client authentication fields to the cloud source form, while deprecating the existing OCM token field. I also did some small refactoring by moving the cloud source form files together and added some help texts.

The OCM client authentication (client ID + client secret) replace the currently used OCM offline tokens. The offline tokens have been deprecated by OCM and will be removed some time next year.

Note that this PR is based on the backend PR #12248.

I will create a docs PR to update the field descriptions once this PR is finalized.

User-facing documentation

• [x] CHANGELOG is updated OR update is not needed
• [ ] documentation PR is created and is linked above OR is not needed

Testing and quality

• [x] the change is production ready: the change is GA or otherwise the functionality is gated by a feature flag
• [x] CI results are inspected

Automated testing

• [ ] added unit tests
• [ ] added e2e tests
• [ ] added regression tests
• [ ] added compatibility tests
• [x] modified existing tests

How I validated my change

see screenshots