Extend StartTask API to allow multiple tasks

Currently the StartTask API allows schedulers to start one task on multiple instances but there is no mechanism to customised the overrides on each instance.

Our company routinely runs massively parallel jobs where the same task is run over and over again with different commands or environment variables. The current limitation of StartTask means our scheduler needs to make an API call for every combination of parameters we want to run, which could be a lot.

We would like to have an API like `start-task task1 with override1 on instance1, override2 on instance2` or equivalently `start-task task-definition1 on instance1, task-definition2 on instance2`.

Start by reading the StartTask API documentation and the examples in this issue. Compare the current one-task, multiple-instance behavior with the requested per-instance task definitions or overrides. Done means schedulers can submit multiple task configurations in one request, with each configuration associated with its target instance.

Written by the indexing model from the issue text.