feat: introduce read preference (#5783)

* feat: introduce read preference

* feat: introduce `RegionQueryHandlerFactory`

* feat: extract ReadPreference from http header

* test: add more tests

* chore: apply suggestions from CR

* chore: apply suggestions from CR

src/partition/Cargo.toml

@@ -20,6 +20,7 @@ datatypes.workspace = true
 itertools.workspace = true
 serde.workspace = true
 serde_json.workspace = true
+session.workspace = true
 snafu.workspace = true
 sql.workspace = true
 sqlparser.workspace = true

src/partition/src/manager.rs

@@ -171,6 +171,7 @@ impl PartitionRuleManager {
         Ok(Arc::new(rule) as _)
     }
 
+    /// Find the leader of the region.
     pub async fn find_region_leader(&self, region_id: RegionId) -> Result<Peer> {
         let region_routes = &self
             .find_physical_table_route(region_id.table_id())