router: add box tuple case for shard key hashing

In scope of tarantool/tarantool#8147, box tuples returned from remote
procedure calls our now encoded as a new extension, `MP_TUPLE`, and,
consequently, the results of such calls are now decoded as box tuples. For
compatibility with this feature, we need to add this case to shard key
hashing. Since box tuples are semantically equivalent to Lua tables, we can
process them the same way.

Needed for tarantool/tarantool#8147

NO_DOC=<refactoring>
NO_TEST=<refactoring>

Author: CuriousGeorgiy

vshard/hash.lua

@@ -39,7 +39,7 @@ end
 -- double.
 --
 local function mpcrc32(shard_key)
-    if type(shard_key) ~= 'table' then
+    if type(shard_key) ~= 'table' and not box.tuple.is(shard_key) then
         return ldigest.crc32(mpcrc32_one(shard_key))
     else
         local crc32 = ldigest.crc32.new()