Allow configuration of yKey for each series of a plot (Overlay)

[shefalijoshi]

Is your feature request related to a problem? Please describe.
Currently when a yKey is set for a plot, it is automatically saved to the first series of a plot (even if the plot is composed of multiple series)

Describe the solution you'd like
When the yKey is saved on a plot, save the yKey to all the series of the plot
https://github.com/nasa/openmct/blob/master/src/plugins/plot/MctPlot.vue#L1101
This will allow the user to later specify different yKeys for each series depending on the y-axis they are assigned to.

Describe alternatives you've considered
None

Additional context
This is the first step to getting multiple Y Axis into plots for Open MCT

[khalidadil]

@shefalijoshi Was there a good way to test this now?