Add ability to specify callbacks to run after each client request

[ml-evs]

This PR adds callbacks to the client, callable functions that will be execute with the results from each query as they come in. This can be useful for constructing secondary databases, saving to files etc.

[codecov]

Codecov Report

Merging #1519 (670c7a5) into master (c87d6c3) will decrease coverage by 0.01%.
The diff coverage is 92.30%.

@@            Coverage Diff             @@
##           master    #1519      +/-   ##
==========================================
- Coverage   91.10%   91.10%   -0.01%     
==========================================
  Files          74       74              
  Lines        4507     4518      +11     
==========================================
+ Hits         4106     4116      +10     
- Misses        401      402       +1     

Flag	Coverage Δ
project	91.10% <92.30%> (-0.01%)	⬇️
validator	90.99% <92.30%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
optimade/client/client.py	82.00% <92.30%> (+0.33%)	⬆️

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[JPBergsma]

LGTM

[ml-evs]

Thanks @JPBergsma -- I want to write some examples into the docs at some point but not sure when I will get time.

I'm going to make a 0.21 release now ahead of the tutorials next week.