[TieredStorage] AccountMetaFlags (#31913)

#### Summary of Changes
This PR implements AccountMetaFlags, which will later be used in
the tiered accounts storage to support optional fields.

#### Test Plan
Unit tests are included in this PR.

runtime/src/tiered_storage.rs

@@ -2,6 +2,7 @@ pub mod byte_block;
 pub mod error;
 pub mod file;
 pub mod footer;
+pub mod meta;
 pub mod mmap_utils;
 
 use crate::tiered_storage::error::TieredStorageError;

runtime/src/tiered_storage/meta.rs

@@ -0,0 +1,72 @@
+#![allow(dead_code)]
+//! The account meta and related structs for the tiered storage.
+
+use modular_bitfield::prelude::*;
+
+/// The struct that handles the account meta flags.
+#[bitfield(bits = 32)]
+#[repr(C)]
+#[derive(Debug, Default, Copy, Clone, Eq, PartialEq)]
+pub(crate) struct AccountMetaFlags {
+    /// whether the account meta has rent epoch
+    has_rent_epoch: bool,
+    /// whether the account meta has account hash
+    has_account_hash: bool,
+    /// whether the account meta has write version
+    has_write_version: bool,
+    /// the reserved bits.
+    reserved: B29,
+}
+
+#[cfg(test)]
+pub mod tests {
+    use crate::tiered_storage::meta::*;
+
+    #[test]
+    fn test_account_meta_flags_new() {
+        let flags = AccountMetaFlags::new();
+
+        assert!(!flags.has_rent_epoch());
+        assert!(!flags.has_account_hash());
+        assert!(!flags.has_write_version());
+        assert_eq!(flags.reserved(), 0u32);
+
+        assert_eq!(
+            std::mem::size_of::<AccountMetaFlags>(),
+            std::mem::size_of::<u32>()
+        );
+    }
+
+    fn verify_flags_serialization(flags: &AccountMetaFlags) {
+        assert_eq!(AccountMetaFlags::from_bytes(flags.into_bytes()), *flags);
+    }
+
+    #[test]
+    fn test_account_meta_flags_set() {
+        let mut flags = AccountMetaFlags::new();
+
+        flags.set_has_rent_epoch(true);
+
+        assert!(flags.has_rent_epoch());
+        assert!(!flags.has_account_hash());
+        assert!(!flags.has_write_version());
+        verify_flags_serialization(&flags);
+
+        flags.set_has_account_hash(true);
+
+        assert!(flags.has_rent_epoch());
+        assert!(flags.has_account_hash());
+        assert!(!flags.has_write_version());
+        verify_flags_serialization(&flags);
+
+        flags.set_has_write_version(true);
+
+        assert!(flags.has_rent_epoch());
+        assert!(flags.has_account_hash());
+        assert!(flags.has_write_version());
+        verify_flags_serialization(&flags);
+
+        // make sure the reserved bits are untouched.
+        assert_eq!(flags.reserved(), 0u32);
+    }
+}