feat(pageserver): add reldir migration configs (#10439)

## Problem

Part of #9516 per RFC at https://github.com/neondatabase/neon/pull/10412

## Summary of changes

Adding the necessary config items and index_part items for the large
relation count work.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

control_plane/src/pageserver.rs

@@ -418,6 +418,11 @@ impl PageServerNode {
                 .map(serde_json::from_str)
                 .transpose()
                 .context("parse `wal_receiver_protocol_override` from json")?,
+            rel_size_v2_enabled: settings
+                .remove("rel_size_v2_enabled")
+                .map(|x| x.parse::<bool>())
+                .transpose()
+                .context("Failed to parse 'rel_size_v2_enabled' as bool")?,
         };
         if !settings.is_empty() {
             bail!("Unrecognized tenant settings: {settings:?}")