Feature: Introducing Static Code Analysis, PHPStan and refactor

[roberto-butti]

Pull request type

Jira Link: INT-

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Other (please describe):

Due to the changes needed to reach level 0 of static code analysis, I'm creating this PR to introduce phpstan and add some refactoring.

The refactor was:

• moving functions that call getStories() from BaseClient to Client;
• moving "enrich" related functions from BaseClient to Client
• moving "relations" related functions from BaseClient to Client
• moving "resolvingLinks" related functions from BaseClient to Client
• declare attributes for classes
• cleaning responseHandler() for avoid duplicate code in the classes

I have some proposals (not yet included in this PR): creating Traits to group methods and attributes related to specific functionality, for example:

• Traits Relatable with: $_relationsList, getResolvedRelations() , insertRelations(),
• Traits Enrichable with: enrichContent(), enrichStories()
• Traits Linkable with: getResolvedLinks(), insertLinks()