Allow polite mailto specification in ETL

Description

Expose the configuration for the crossref client that the ETL uses to alert crossref API to who is using it. Allows access to better pools.

[x] I have tested these changes locally and they work as expected.
[x] I have added or updated tests to cover any new functionality or bug fixes.
[x] I have updated the documentation to reflect any changes or additions to the project.
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