Add ability to change grid widths of provider sub-widgets

[CasperWA]

Add width_ratio and width_space parameters to OptimadeQueryProviderWidget.
These parameters allows the user to specify the number of grid columns used to layout the dropdown and informational sub-widgets.
width_space indicates how many columns should be spaced in between the two sub-widgets.

The default is width_ratio=(10, 21) and width_space=1, meaning the total number of columns are 31 (sum of width_ratio) and the dropdown sub-widget spans 10 columns, while the informational sub-widget spans 20 columns (21 - the value of width_space, which is 1).

[codecov]

Codecov Report

Merging #188 into develop will decrease coverage by 0.01%.
The diff coverage is 30.76%.

@@             Coverage Diff             @@
##           develop     #188      +/-   ##
===========================================
- Coverage    35.99%   35.98%   -0.02%     
===========================================
  Files           17       17              
  Lines         2128     2137       +9     
===========================================
+ Hits           766      769       +3     
- Misses        1362     1368       +6     

Flag	Coverage Δ
#optimade-client	35.98% <30.76%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
optimade_client/query_provider.py	35.89% <30.76%> (-0.77%)	⬇️

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