TieredStorage struct (1/N) -- new_writable() (#32538)

#### Summary of Changes
This PR initiates the implementation of the main struct for the
tiered accounts storage --- TieredStorage.  Specifically,
it defines the TieredStorage struct, TieredStorageFormat,
and skeleton implementation of new_writable().

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

runtime/src/tiered_storage.rs

@@ -1,3 +1,5 @@
+#![allow(dead_code)]
+
 pub mod byte_block;
 pub mod error;
 pub mod file;
@@ -8,6 +10,60 @@ pub mod meta;
 pub mod mmap_utils;
 pub mod readable;
 
-use crate::tiered_storage::error::TieredStorageError;
+use {
+    error::TieredStorageError,
+    footer::{AccountBlockFormat, AccountMetaFormat, OwnersBlockFormat},
+    index::AccountIndexFormat,
+    std::path::{Path, PathBuf},
+};
 
 pub type TieredStorageResult<T> = Result<T, TieredStorageError>;
+
+/// The struct that defines the formats of all building blocks of a
+/// TieredStorage.
+#[derive(Clone, Debug)]
+pub struct TieredStorageFormat {
+    pub meta_entry_size: usize,
+    pub account_meta_format: AccountMetaFormat,
+    pub owners_block_format: OwnersBlockFormat,
+    pub account_index_format: AccountIndexFormat,
+    pub account_block_format: AccountBlockFormat,
+}
+
+#[derive(Debug)]
+pub struct TieredStorage {
+    format: Option<TieredStorageFormat>,
+    path: PathBuf,
+}
+
+impl TieredStorage {
+    /// Creates a new writable instance of TieredStorage based on the
+    /// specified path and TieredStorageFormat.
+    ///
+    /// Note that the actual file will not be created until append_accounts
+    /// is called.
+    pub fn new_writable(path: impl Into<PathBuf>, format: TieredStorageFormat) -> Self {
+        Self {
+            format: Some(format),
+            path: path.into(),
+        }
+    }
+
+    /// Returns the path to this TieredStorage.
+    pub fn path(&self) -> &Path {
+        self.path.as_path()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use {super::*, hot::HOT_FORMAT};
+
+    #[test]
+    fn test_new_writable() {
+        let path = PathBuf::default();
+        let ts = TieredStorage::new_writable(&path, HOT_FORMAT.clone());
+
+        assert_eq!(ts.path(), path);
+    }
+}

runtime/src/tiered_storage/hot.rs

@@ -6,7 +6,10 @@ use {
         account_storage::meta::StoredMetaWriteVersion,
         tiered_storage::{
             byte_block,
+            footer::{AccountBlockFormat, AccountMetaFormat, OwnersBlockFormat},
+            index::AccountIndexFormat,
             meta::{AccountMetaFlags, AccountMetaOptionalFields, TieredAccountMeta},
+            TieredStorageFormat,
         },
     },
     modular_bitfield::prelude::*,
@@ -14,6 +17,14 @@ use {
     std::option::Option,
 };
 
+pub const HOT_FORMAT: TieredStorageFormat = TieredStorageFormat {
+    meta_entry_size: std::mem::size_of::<HotAccountMeta>(),
+    account_meta_format: AccountMetaFormat::Hot,
+    owners_block_format: OwnersBlockFormat::LocalIndex,
+    account_index_format: AccountIndexFormat::AddressAndOffset,
+    account_block_format: AccountBlockFormat::AlignedRaw,
+};
+
 /// The maximum number of padding bytes used in a hot account entry.
 const MAX_HOT_PADDING: u8 = 7;