storcon: add API definitions for exclude_timeline and term_bump (#11197)

Adds API definitions for the safekeeper API endpoints `exclude_timeline`
and `term_bump`. Also does a bugfix to return the correct type from
`delete_timeline`.

Part of #8614

libs/safekeeper_api/src/models.rs

@@ -221,6 +221,11 @@ pub struct TimelineMembershipSwitchResponse {
     pub current_conf: Configuration,
 }
 
+#[derive(Clone, Copy, Serialize, Deserialize)]
+pub struct TimelineDeleteResult {
+    pub dir_existed: bool,
+}
+
 fn lsn_invalid() -> Lsn {
     Lsn::INVALID
 }