feat: add api to get information about diskspace usage of database. (jsonrpc method: get_storage_usage_report_string) (#7486)

new jsonrpc api: `get_storage_usage_report_string(accountId)`
new rust API: `get_storage_usage(&context)`

src/lib.rs

@@ -89,6 +89,7 @@ pub mod securejoin;
 mod simplify;
 mod smtp;
 pub mod stock_str;
+pub mod storage_usage;
 mod sync;
 mod timesmearing;
 mod token;