Worker management overhaul

Description

In line with the new worker management design, the workers and core have been reworked. Resolves #121 (closed) #125 (closed) #124 (closed)

Changes

• Removed the local worker spawner
• Workers are now exclusively single-threaded

Additions

• The core reports the number of workers in the ping debug message
• Workers report to the core if they've been terminated, so they are properly removed from the pool
• Worker docker image
• Added endpoint to retrieve a worker token, since after removing the local worker spawner this would not have been possible otherwise.

Test and Review

• Tested the new API endpoint

To be filled in by the reviewers

• All of the methods are commented to expectation

• The methods are tested to satisfaction

• There are no unnecessary files present in the MR

• The continuous integration has no problems with the MR

• The MR is filled in as requested (including labels, milestones, and reviewers)

• The documentation is up-to-date

• All nullable parameters are marked as such