Download SLRU segments on demand (#6151)

## Problem

See https://github.com/neondatabase/cloud/issues/8673

## Summary of changes


Download missed SLRU segments from page server

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

---------

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>
Co-authored-by: Heikki Linnakangas <heikki@neon.tech>

pageserver/client/src/page_service.rs

@@ -156,7 +156,8 @@ impl PagestreamClient {
             PagestreamBeMessage::Error(e) => anyhow::bail!("Error: {:?}", e),
             PagestreamBeMessage::Exists(_)
             | PagestreamBeMessage::Nblocks(_)
-            | PagestreamBeMessage::DbSize(_) => {
+            | PagestreamBeMessage::DbSize(_)
+            | PagestreamBeMessage::GetSlruSegment(_) => {
                 anyhow::bail!(
                     "unexpected be message kind in response to getpage request: {}",
                     msg.kind()