Store read_only flag in pageserver timelines

[arpad-m]

It is useful also for the pageserver to store whether a timeline was read_only or not: we can stop/not run internal processes, not look for safekeepers at all, etc. Also, for the outside it is nicer if the TimelineInfo contains whether a timeline is read only or not.

So introduce a new read_only field to index-part.json. Currently, it's a one time choice one can't change (in either direction), and read-only property is non-recursive. Also, we only support it for branch timelines, so not for root timelines or imported timelines.

Of course this only applies to new timelines, so the existing snapshots don't have this applied.

Follow-up of #12015

[github-actions]

No tests were run or test report is not available

Test coverage report is not available

_{The comment gets automatically updated with the latest test results
f0c63fa at 2025-07-09T18:11:06.550Z :recycle:}