Add BaseSearchRequestTemplatePage, change SearchRequestTemplatePage

[Gallaecio]

This is a backward-incompatible change that makes things more consistent with other, similar classes, and gives both classes access to the no_item_found() method from _BasePage.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 0.00%. Comparing base (9136859) to head (756029e).
Report is 40 commits behind head on main.

Files with missing lines	Patch %	Lines
zyte_common_items/pages/search_request_template.py	0.00%	3 Missing ⚠️
zyte_common_items/pages/__init__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #113   +/-   ##
=====================================
  Coverage   0.00%   0.00%           
=====================================
  Files         58      60    +2     
  Lines       2236    2285   +49     
=====================================
- Misses      2236    2285   +49     

Files with missing lines	Coverage Δ
zyte_common_items/__init__.py	0.00% <ø> (ø)
zyte_common_items/pages/__init__.py	0.00% <0.00%> (ø)
zyte_common_items/pages/search_request_template.py	0.00% <0.00%> (ø)

... and 6 files with indirect coverage changes