Add another remote-data directive, that works for the whole file/rest of the file

 Would be nice to add another directive, that works for the whole file/rest of the file and not just for a code block. e.g. `doctest-remote-data-all`

_Originally posted by @bsipocz in https://github.com/astropy/pytest-doctestplus/issues/137#issuecomment-749886094_