Allow image.decode() to take in an image mode to create a fixed mode image column

[samster25]

• imagine we are decoding a column of images that have mixed modes such as RGB and grayscale, the user may want to pass in a fixed mode so we convert everything to a fixed type

something like

df['x'].image.decode(mode"rgb")

[jaychia]

Definitely a painpoint, I'm running into these issues when attempting to decode a bunch of images:

Seems like our current code just gives up when decoding heterogenous images. We should probably attempt to coerce them into the same mode and dtype