[TASK] Implement readonly site settings

In preparation for readonly system settings, settings
definitions now suppot a readonly attribute.

Releases: main
Resolves: #104919
Related: #102799
Change-Id: I4ea22bf7d3f93b895493291993a01b8bf5695275
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/86028


Reviewed-by: Sebastian Iffland <sebastian.iffland@fullhaus.de>
Reviewed-by: Andreas Kienast <a.fernandez@scripting-base.de>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: Andreas Kienast <a.fernandez@scripting-base.de>
Reviewed-by: Frank Nägler <frank.naegler@typo3.com>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>