secure-storage: enable calling KVStorage::get for Namespaced<&S>
secure-storage: enable calling KVStorage::get for Namespaced<&S>

In order to work around the fact that `&S` does not implement
`KVStorage`, lets add an explicit `get` method in order to enable
calling `KVStorage::get` with `Namespaced<&S>`.

libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core