[CDF-25262] ⬆ Update agents API (#1763)

# Description


The Cognite-SDK now has an implementation of the agents API, so we no
longer need the one we have in Toolkit for early adapters.

**Note** The current Toolkit implementation sets the header incorrectly
which causes the API calls to be rejected. Switching to the cognite-sdk
implementation sets the header correctly.

## Changelog

- [x] Patch
- [ ] Minor
- [ ] Skip

## cdf

### Fixed

- [alpha] Running `cdf deploy` with agents no longer fails.

## templates

No changes.
