Add existing_initdb_timeline_id param to timeline creation (#5912)

This PR adds an `existing_initdb_timeline_id` option to timeline
creation APIs, taking an optional timeline ID.

Follow-up of  #5390.

If the `existing_initdb_timeline_id` option is specified via the HTTP
API, the pageserver downloads the existing initdb archive from the given
timeline ID and extracts it, instead of running initdb itself.

---------

Co-authored-by: Christian Schwarz <christian@neon.tech>

pageserver/src/http/openapi_spec.yml

@@ -1028,6 +1028,9 @@ paths:
                   format: hex
                 pg_version:
                   type: integer
+                existing_initdb_timeline_id:
+                  type: string
+                  format: hex
       responses:
         "201":
           description: TimelineInfo

pageserver/src/http/routes.rs

@@ -441,6 +441,7 @@ async fn timeline_create_handler(
             request_data.ancestor_timeline_id.map(TimelineId::from),
             request_data.ancestor_start_lsn,
             request_data.pg_version.unwrap_or(crate::DEFAULT_PG_VERSION),
+            request_data.existing_initdb_timeline_id,
             state.broker_client.clone(),
             &ctx,
         )