Serial and parallel execution

TLDR;

Opctl supports using serial and/or parallel statements to make parts of your op run serially (one by one in order) or in parallel (all at once without order).

Example

1. Start this op:

   name: serialAndParallelExecution
   run:
     serial:
       - parallel:
           - container:
               cmd: [echo, "parallel[0]"]
               image: { ref: alpine }
           - container:
               cmd: [echo, "parallel[1]"]
               image: { ref: alpine }
       - serial:
           - container:
               cmd: [echo, "serial[0]"]
               image: { ref: alpine }
           - container:
               cmd: [echo, "serial[1]"]
               image: { ref: alpine }
2. Observe:
   1. for the parallel statement, containers are run in parallel (all at once without order)
   2. for the serial statement, containers are run serially (one by one in order)