Implement `parallelism` for `template`s

[moshewe]

Summary

The current implementation does not allow creating templates specifying the parallelism field as per the Argo spec.

Use Cases

Many possible use cases, my current one is limiting the amount of parallel nodes running a certain template so as to save resources at the expense of calculation time, i.e. run less instances of a certain process in parallel to save cost while increasing calculation time.

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[tachyus-ryan]

@moshewe would semaphoreRef be a reasonable alternative?