Add proper export (to bibtex)

[claell]

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pybliometrics version: 3.5.2

Affected classes: AbstractRetrieval

Expected behavior: My use case for this project is to ideally automate workflows that I have to otherwise use the Scopus web UI for. For a literature review, I want to search Scopus and export the results (ideally to bibtex). This is possible through the web UI. Unfortunately, it doesn't seem fully supported with this package (https://pybliometrics.readthedocs.io/en/stable/classes/AbstractRetrieval.html#pybliometrics.scopus.AbstractRetrieval.get_bibtex is limited to Journal articles right now). Is this due to API limitations?

I saw this implemented in another project, but maybe they just used their own code to achieve the behavior: https://github.com/muschellij2/rscopus/issues/20.