Add ISiteSearch interface

[pdelboca]

This PR adds a new ISiteSearch to allow users to hook specific logic before/after each type of search.

This is inspired by before_package_search and after_package_search methods implemented in IPackageController:

[codecov-commenter]

Codecov Report

Merging #2 (26d6dbd) into main (2e2de81) will increase coverage by 0.16%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##             main       #2      +/-   ##
==========================================
+ Coverage   91.13%   91.29%   +0.16%     
==========================================
  Files          16       18       +2     
  Lines        1150     1286     +136     
==========================================
+ Hits         1048     1174     +126     
- Misses        102      112      +10     

Impacted Files	Coverage Δ
ckanext/sitesearch/interfaces.py	54.54% <54.54%> (ø)
ckanext/sitesearch/logic/action.py	100.00% <100.00%> (ø)
ckanext/sitesearch/tests/test_interfaces.py	100.00% <100.00%> (ø)

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[amercader]

Looks good @pdelboca ! Only two minor comments. Also please add a small mention in the README about the interface (you can link to interfaces.py for the documentation) and run black on all files before pushing, thanks!

[pdelboca]

Done @amercader !

I do not have permissions to merge.

[amercader]

@pdelboca thanks, you should have perms now