[ENH]: secondary_x/yaxis accept transform argument

### Problem

#25117 wanted to put an extra axes at y=0.  It should be possible to pass `ax.transData` to `secondary_xaxis` so that the location variable is understood as data co-ordinates, not axes-relative co-ordinates.  

### Proposed solution

`secondary_xaxis(0, transform=ax.transData)` should put the secondary axes at y=0.  Of course the default should be ax.transAxes for back-compatibility.  