Fix inconsistency in fetch content signature

[marcospassos]

Summary

This PR introduces a BC break to correct an inconsistency in the fetchContent signature, aligning it with the API of all other SDKs.

Although the PR is valid as it is, it would be preferable to wait for the release of React SDK and import the FetchResponse from it instead of the Plug.

The type is exported in this PR:
croct-tech/plug-js#296

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

[pkg-pr-new]

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npm i https://pkg.pr.new/@croct/plug-next@38

commit: 05901b7