Add archival_config endpoint to pageserver (#8414)

This adds an archival_config endpoint to the pageserver. Currently it
has no effect, and always "works", but later the intent is that it will
make a timeline archived/unarchived.

- [x] add yml spec
- [x] add endpoint handler

Part of https://github.com/neondatabase/neon/issues/8088

libs/pageserver_api/src/models.rs

@@ -651,6 +651,17 @@ pub struct TenantDetails {
     pub timelines: Vec<TimelineId>,
 }
 
+#[derive(Serialize, Deserialize, PartialEq, Eq, Clone, Copy, Debug)]
+pub enum TimelineArchivalState {
+    Archived,
+    Unarchived,
+}
+
+#[derive(Serialize, Deserialize, PartialEq, Eq, Clone)]
+pub struct TimelineArchivalConfigRequest {
+    pub state: TimelineArchivalState,
+}
+
 /// This represents the output of the "timeline_detail" and "timeline_list" API calls.
 #[derive(Debug, Serialize, Deserialize, Clone)]
 pub struct TimelineInfo {