[enh] Add Springer Nature engine

Springer Nature is a global publisher dedicated to providing service to research
community [1] with official API [2].

To test this PR, first get your API key following this page:

   https://dev.springernature.com/signup

In searx/engines/springer.py at line 24, add this API key.  I left my own key,
commented out in the line aboce.  Feel free to use it, if needed.

[1] https://www.springernature.com/
[2] https://dev.springernature.com/