read-only timelines (#12015)

Support timeline creations on the storage controller to opt out from
their creation on the safekeepers, introducing the read-only timelines
concept. Read only timelines:

* will never receive WAL of their own, so it's fine to not create them
on the safekeepers
* the property is non-transitive. children of read-only timelines aren't
neccessarily read-only themselves.

This feature can be used for snapshots, to prevent the safekeepers from
being overloaded by empty timelines that won't ever get written to. In
the current world, this is not a problem, because timelines are created
implicitly by the compute connecting to a safekeeper that doesn't have
the timeline yet. In the future however, where the storage controller
creates timelines eagerly, we should watch out for that.

We represent read-only timelines in the storage controller database so
that we ensure that they never touch the safekeepers at all. Especially
we don't want them to cause a mess during the importing process of the
timelines from the cplane to the storcon database.

In a hypothetical future where we have a feature to detach timelines
from safekeepers, we'll either need to find a way to distinguish the
two, or if not, asking safekeepers to list the (empty) timeline prefix
and delete everything from it isn't a big issue either.

This patch will unconditionally hit the new safekeeper timeline creation
path for read-only timelines, without them needing the
`--timelines-onto-safekeepers` flag enabled. This is done because it's
lower risk (no safekeepers or computes involved at all) and gives us
some initial way to verify at least some parts of that code in prod.

https://github.com/neondatabase/cloud/issues/29435
https://github.com/neondatabase/neon/issues/11670

control_plane/src/bin/neon_local.rs

@@ -1279,6 +1279,7 @@ async fn handle_timeline(cmd: &TimelineCmd, env: &mut local_env::LocalEnv) -> Re
                 mode: pageserver_api::models::TimelineCreateRequestMode::Branch {
                     ancestor_timeline_id,
                     ancestor_start_lsn: start_lsn,
+                    read_only: false,
                     pg_version: None,
                 },
             };