Feature sitemap txt

[kennethrioja]

Fixes #1118

Summary of changes

• Added a way to parse sitemap.txt (which are urls separated by \n)
• Added a small summary of the URLs where Bioschemas are not found

Motivation and context

• One of the source I will register has a sitemap in a .txt file extension and not .xml (see Google documentation).
• I know some of the URLs in my source do not have bioschemas, so I wanted to have a clear list of them all (see screenshot)

Screenshots

Checklist

• I have read and followed the CONTRIBUTING guide.
• I confirm that I have the authority necessary to make this contribution on behalf of its copyright owner and agree
  to license it to the TeSS codebase under the
  BSD license.

[fbacall]

Code looks good - could you add a test for this?

Here's an example of a test for the XML sitemap: https://github.com/ElixirTeSS/TeSS/blob/master/test/unit/ingestors/bioschemas_ingestor_test.rb#L39-L97

It does not have to be as complex as that though, just something simple that will trigger the txt sitemap parsing code