3.16 improvement - Make the beacon script output ready-to-store in the DB to avoid additional formatting in PHP

[MathieuLamiot]

Context
Sub-task #6449
The format expected to be stored in the DB for LCP/ATF is quite different from the format of the data provided by the beacon script. Because of this, we have some formatting logic in add_lcp_data, typically the handling of $viewport.

Expected behavior
Make the output of the beacon script ready to be stored in the DB without further processing and formatting, apart from security checks in PHP.

Acceptance Criteria
NRT on 3.16.