Amend default kwarg behaviour to only over-write specific kwarg in Display classes

[lucyleeow]

Previously, if you passed any curve_kwargs, it would over-ride all default kwargs.

Now, if an individual curve_kwargs that changes the same parameter as a default kwarg is passed, only that parameter will be over-ridden. All other default kwargs will still be used. E.g., if the user set color to red in curve_kwargs, only the defualt color parameter will be over-ridden. The other parameters (e.g., "alpha": 0.5, "linestyle": "--") will still be used.

I think it is more likely that if a user e.g., sets the curve color to be red, they still want the other default kwargs (i.e., they only want to change the color).
In particular, I changed this because it is necessary for precision recall as I think we always want the default "drawstyle": "steps-post" (to prevent interpolation), unless the user specifically changes it.

As discussed in # 30508 this should be separated out into it's own PR.