Add test_explicit_timeline_creation_storcon and make it work (#11261)

Adds a basic test that makes the storcon issue explicit creation of a
timeline on safeekepers (main storcon PR in #11058). It was adapted from
`test_explicit_timeline_creation` from #11002.

Also, do a bunch of fixes needed to get the test work (the API
definitions weren't correct), and log more stuff when we can't create a
new timeline due to no safekeepers being active.

Part of #9011

---------

Co-authored-by: Arseny Sher <sher-ars@yandex.ru>

libs/safekeeper_api/src/models.rs

@@ -23,6 +23,7 @@ pub struct TimelineCreateRequest {
     pub tenant_id: TenantId,
     pub timeline_id: TimelineId,
     pub mconf: Configuration,
+    /// In the PG_VERSION_NUM macro format, like 140017.
     pub pg_version: u32,
     pub system_id: Option<u64>,
     // By default WAL_SEGMENT_SIZE