[TieredAccountStorage] TieredReadableAccount (#32380)

#### Summary of Changes
This PR introduces TieredReadableAccount, a struct that takes a generic
TieredAccountMeta that implements ReadableAccount and support
StoredAccountMeta.

#### Test Plan
More tests will be added once more reader and writer functions for hot
accounts have been added.

runtime/src/tiered_storage.rs

@@ -5,6 +5,7 @@ pub mod footer;
 pub mod hot;
 pub mod meta;
 pub mod mmap_utils;
+pub mod readable;
 
 use crate::tiered_storage::error::TieredStorageError;
 

runtime/src/tiered_storage/readable.rs

@@ -0,0 +1,78 @@
+use {
+    crate::{
+        account_storage::meta::StoredMetaWriteVersion, tiered_storage::meta::TieredAccountMeta,
+    },
+    solana_sdk::{account::ReadableAccount, hash::Hash, pubkey::Pubkey, stake_history::Epoch},
+};
+
+/// The struct that offers read APIs for accessing a TieredAccount.
+#[derive(PartialEq, Eq, Debug)]
+pub struct TieredReadableAccount<'a, M: TieredAccountMeta> {
+    /// TieredAccountMeta
+    pub(crate) meta: &'a M,
+    /// The address of the account
+    pub(crate) address: &'a Pubkey,
+    /// The address of the account owner
+    pub(crate) owner: &'a Pubkey,
+    /// The index for accessing the account inside its belonging AccountsFile
+    pub(crate) index: usize,
+    /// The account block that contains this account.  Note that this account
+    /// block may be shared with other accounts.
+    pub(crate) account_block: &'a [u8],
+}
+
+impl<'a, M: TieredAccountMeta> TieredReadableAccount<'a, M> {
+    /// Returns the address of this account.
+    pub fn address(&self) -> &'a Pubkey {
+        self.address
+    }
+
+    /// Returns the hash of this account.
+    pub fn hash(&self) -> Option<&'a Hash> {
+        self.meta.account_hash(self.account_block)
+    }
+
+    /// Returns the index to this account in its AccountsFile.
+    pub fn index(&self) -> usize {
+        self.index
+    }
+
+    /// Returns the write version of the account.
+    pub fn write_version(&self) -> Option<StoredMetaWriteVersion> {
+        self.meta.write_version(self.account_block)
+    }
+}
+
+impl<'a, M: TieredAccountMeta> ReadableAccount for TieredReadableAccount<'a, M> {
+    /// Returns the balance of the lamports of this account.
+    fn lamports(&self) -> u64 {
+        self.meta.lamports()
+    }
+
+    /// Returns the address of the owner of this account.
+    fn owner(&self) -> &'a Pubkey {
+        self.owner
+    }
+
+    /// Returns true if the data associated to this account is executable.
+    ///
+    /// Temporarily unimplemented!() as program runtime v2 will use
+    /// a different API for executable.
+    fn executable(&self) -> bool {
+        unimplemented!();
+    }
+
+    /// Returns the epoch that this account will next owe rent by parsing
+    /// the specified account block.  Epoch::MAX will be returned if the account
+    /// is rent-exempt.
+    fn rent_epoch(&self) -> Epoch {
+        self.meta
+            .rent_epoch(self.account_block)
+            .unwrap_or(Epoch::MAX)
+    }
+
+    /// Returns the data associated to this account.
+    fn data(&self) -> &'a [u8] {
+        self.meta.account_data(self.account_block)
+    }
+}