feat: add `countIn` operation

[drupol]

This PR:

• [x] Provide a new operation countIn inspired from https://github.com/fiiSoft/fiisoft-jackdaw-stream/ project
• [ ] It breaks backward compatibility
• [x] Is covered by unit tests
• [x] Has static analysis tests (psalm, phpstan)
• [x] Has documentation
• [ ] Is an experimental thing

[what-the-diff[bot]]

PR Summary

• Introduction of 'countIn' Operation A new file has been added that contains the vital code for conducting a 'countIn' operation. This technique enables us to determine how many items in a specific collection meet a certain criterion.

• 'CountInAble' Interface An outline, or 'interface', for the 'countIn' operation has been provided. The role of this interface is to guide other parts of our system on how this operation should be done.

• Updating the 'Collection' and 'CollectionDecorator' Classes The 'Collection' and 'CollectionDecorator' parts of our system have been adjusted to embrace the new 'countIn' operation. They are now built in a way that they understand and can conduct this operation.

• Modification of 'Contract/Collection' Interface Our pre-existing 'Contract/Collection' interface now extends the newly introduced 'CountInAble' interface, ensuring that it's focusing on relationships and operations vital to the 'CountInAble' operation.

• Unit Tests for 'countIn' Operation We have incorporated unit tests for the 'countIn' operation. Unit tests serve to verify that each unit of our software (in this case, the 'countIn' operation) is functioning as expected.

• Documentation Updates Detailed information about the 'CountInAble' operation can now be found in our official API documentation. The updates include a reference in the 'Interface' section and numerous links to the 'CountInAble' interface in the 'See Also' sections.

[drupol]

Thanks for the review :)