api: allow setting the preset id on a workspace build when a user creates a new workspace

Once a user has chosen a preset and submits the form to create a workspace, the preset_id should be set on the resulting entry in the `workspace_builds` table. It is not yet clear which endpoints will have to be modified for this, but those can be found by creating a workspace from the web UI and inspecting the network traffic. These endpoints might not directly modify the `workspace_builds` table. It is possible that there is some chain through which the preset_id will have to pass to finally reach the query that sets it in the database.

Acceptance Criteria:
* The minimal set of endpoints required to set the preset ID for a new workspace build is modified optionally accept such a parameter.
* All existing tests pass without significant modification, as existing behaviour without presets should remain unaffected.
* Unit tests to show that if a preset ID is provided, then the value is correctly set in the database.