key_def: Introduce `__len` metamethod

The metamethod is a way to key_def length introspection.

Closes #10111

@TarantoolBot document
Title: key_def length introspection

To check key_def length (parts count) there is a standard lua operator
`#` (`__len` metamethod).

Example:

```lua
function is_full_pkey(space, key)
    return #space.index[0].parts == #key
end
```