Make `_array_to_b64str` part of the public API (and find a better name)

[emmanuelle]

The function _array_to_b64str defined in the _imshow.py module (https://github.com/plotly/plotly.py/blob/master/packages/python/plotly/plotly/express/_imshow.py#L27) is quite useful in Dash apps with image processing. We could make it part of the public API (where ? probably in plotly.utils ?). What do you think @nicolaskruchten ? This was brought up recently in Almar's work (plotly/dash-slicer#2).

[gvwilson]

Hi - we are trying to tidy up the stale issues and PRs in Plotly's public repositories so that we can focus on things that are still important to our community. Since this one has been sitting for several years, I'm going to close it; if it is still a concern, please add a comment letting us know what recent version of our software you've checked it with so that I can reopen it and add it to our backlog. Thanks for your help - @gvwilson