[UX][DX] provide utility functions to retrieve field labels

As a contrib module, I want to provide a list of fields on my admin settings form. I have the field machine names, but in order to provide a better user experience, I'd like to provide the human-readable names instead.

I'd like a helper function, something like field_get_label() that would provide the most commonly used label across all instances. I'd also like the matching field_get_labels() that would provide me with a list of all labels for that field, perhaps keyed by bundle name.

edit: The latest PR also includes a field_get_all_labels() to get labels for more than one field at a time.

I've done a crazy loop-de-loop in the nodequeue module to get the field labels, but would prefer to have something in core so that every contrib module does not need to do the same.

I expect that if contrib developers are faced with this much work to get a field label, they are likely to print the machine name instead, and be done with it. By providing a utility function we decrease the burden on our contrib developers and are also likely to create a better experience for our end users!

PR: https://github.com/backdrop/backdrop/pull/2552