Add isomorphic content API utilities

[marcospassos]

Summary

This pull request introduces the low-level content APIs that were originally added to the plug-react library, but due to its broad scope, it makes more sense to belong to the plug-js, which is the foundation library for all Javascript-based projects.

The API changes slightly from the original implementation – which was intended for server-side only – to support both client and server-side requests as this better aligns with this library scope.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[denis-rossati]

Further to everyone's comments, lgtm

[Fryuni]

Noice