[FEATURE] Split `get-osmdata.R` into several files

[mpadge]

@jmaspons Once we've merged your current PRs (#299, #302, #303), the get-osmdata.R file should be split into several smaller files. I suggest:

• get-osmdata.R (with only the minor helper functions)
• get-osmdata-sf.R
• get-osmdata-sp.R
• get-osmdata-sp.R
• get-osmdata-df.R
• get-osmdata-meta.R

But we can see what works best once we get that far. You may assign yourself here if you're keen to do that, otherwise I'll happily do it, and ask you to review.

[jmaspons]

Ok! I'll try after #299 is merged, which is ready. I can't assign myself for the task. Can you add me, @mpadge ?