Add API for safekeeper timeline copy (#6091)

Implement API for cloning a single timeline inside a safekeeper. Also
add API for calculating a sha256 hash of WAL, which is used in tests.

`/copy` API works by copying objects inside S3 for all but the last
segments, and the last segments are copied on-disk. A special temporary
directory is created for a timeline, because copy can take a lot of
time, especially for large timelines. After all files segments have been
prepared, this directory is mounted to the main tree and timeline is
loaded to memory.

Some caveats:
- large timelines can take a lot of time to copy, because we need to
copy many S3 segments
- caller should wait for HTTP call to finish indefinetely and don't
close the HTTP connection, because it will stop the process, which is
not continued in the background
- `until_lsn` must be a valid LSN, otherwise bad things can happen
- API will return 200 if specified `timeline_id` already exists, even if
it's not a copy
- each safekeeper will try to copy S3 segments, so it's better to not
call this API in-parallel on different safekeepers

libs/safekeeper_api/src/models.rs

@@ -51,3 +51,9 @@ pub struct SkTimelineInfo {
     #[serde(default)]
     pub http_connstr: Option<String>,
 }
+
+#[derive(Debug, Clone, Deserialize, Serialize)]
+pub struct TimelineCopyRequest {
+    pub target_timeline_id: TimelineId,
+    pub until_lsn: Lsn,
+}