Make ShredStorageType::RocksLevel public (#23272)

#### Summary of Changes
This PR adds two hidden arguments to the validator that allow users to use RocksDB's FIFO compaction for storing shreds.

        --shred-storage <SHRED_STORAGE>
            EXPERIMENTAL: Controls how RocksDB compacts shreds.  *WARNING*: You will lose your ledger data
            when you switch between options. Possible values are: 'level': stores shreds using RocksDB's default (level)
            compaction. 'fifo': stores shreds under RocksDB's FIFO compaction. This option is more efficient on
            disk-write-bytes of the ledger store. [default: level]  [possible values: level, fifo]

        --shred-storage-size <SHRED_STORAGE_SIZE_BYTES>
            The shred storage size in bytes. The suggested value is 50% of your ledger storage size in bytes. [default:
            268435456000]

ledger/src/blockstore_db.rs

@@ -35,6 +35,11 @@ use {
     thiserror::Error,
 };
 
+// The default storage size for storing shreds when `rocksdb-shred-compaction`
+// is set to `fifo` in the validator arguments.  This amount of storage size
+// in bytes will equally allocated to both data shreds and coding shreds.
+pub const DEFAULT_ROCKS_FIFO_SHRED_STORAGE_SIZE_BYTES: u64 = 250 * 1024 * 1024 * 1024;
+
 const MAX_WRITE_BUFFER_SIZE: u64 = 256 * 1024 * 1024; // 256MB
 const FIFO_WRITE_BUFFER_SIZE: u64 = 2 * MAX_WRITE_BUFFER_SIZE;
 // Maximum size of cf::DataShred.  Used when `shred_storage_type`