Add `yaxis_format` to `mpf.plot()` and to `mpf.make_addplot()`

Add `yaxis_format` kwarg to `mpf.plot()` and to `mpf.make_addplot()`

Inspired by [this answer to stackoverflow 67232404](https://stackoverflow.com/questions/67232404/matplotib-finance-mplfinance-formatting-axes-of-chart-unsing-mpf-plot/67233937#67233937) which shows a workaround for formatting the y-axis.

May need two kwargs: `yaxis_format` and `secondary_yaxis_format` or some other way to indicate which y-axis is to be formatted.

Locate the definitions of mpf.plot() and mpf.make_addplot(), then inspect how their axes and secondary axes are currently configured. Review the linked Stack Overflow workaround for the intended formatting behavior. Done means both entry points support y-axis formatting with an explicit, documented behavior for primary and secondary axes.

Written by the indexing model from the issue text.