nasa_ads: Add support for the export service

The Jupyter notebook below includes some examples of how the ADS export service can be used to output the search results in different formats:

https://github.com/adsabs/adsabs-dev-api/blob/master/Export_API.ipynb

Start with the nasa_ads entry point and review the linked Export_API.ipynb for the ADS export service examples and supported output formats. Compare those examples with the existing nasa_ads support; done means the export service is available for the formats and search-result use cases described in the notebook.

Written by the indexing model from the issue text.