[TieredStorage] TieredReadableAccount::data() (#32433)

#### Problem
While TieredReadableAccount implements ReadableAccount that
already includes `fn data(&self) -> &'a [u8]`, a `data()` function that
directly under TieredReadableAccount is still needed in order to
correctly link the lifetime of the returned value and its member
account_block.  Otherwise, cargo will complain lifetime may not
live long enough.

#### Summary of Changes
This PR adds TieredReadableAccount::data() that directly links the lifetime
of its account_block to the returned value.

runtime/src/tiered_storage/readable.rs

@@ -41,6 +41,11 @@ impl<'a, M: TieredAccountMeta> TieredReadableAccount<'a, M> {
     pub fn write_version(&self) -> Option<StoredMetaWriteVersion> {
         self.meta.write_version(self.account_block)
     }
+
+    /// Returns the data associated to this account.
+    pub fn data(&self) -> &'a [u8] {
+        self.meta.account_data(self.account_block)
+    }
 }
 
 impl<'a, M: TieredAccountMeta> ReadableAccount for TieredReadableAccount<'a, M> {
@@ -73,6 +78,6 @@ impl<'a, M: TieredAccountMeta> ReadableAccount for TieredReadableAccount<'a, M>
 
     /// Returns the data associated to this account.
     fn data(&self) -> &'a [u8] {
-        self.meta.account_data(self.account_block)
+        self.data()
     }
 }