lua/key_def: add key validation and comparison functions

Closes #9863

@TarantoolBot document
Title: Document key validation and comparison `key_def` module functions

The following new functions were introduced to the `key_def` Lua module:

 - `key_def:validate_key(key)`: validates a key against a key definition
   object. Raises an exception if the key doesn't match. Returns nothing
   on success. See also `box_key_def_validate_key` C API function.

 - `key_def:validate_full_key(key)`: validates a full key against a key
   definition object. Raises an exception if the key doesn't match.
   Returns nothing on success. See also `box_key_def_validate_full_key`
   C API function.

 - `key_def:validate_tuple(tuple)`: validates a tuple against a key
   definition object. Raises an exception if the tuple doesn't match.
   Returns nothing on success. See also `box_key_def_validate_tuple`
   C API function.

 - `key_def:compare_keys(key_a, key_b)`: compares two keys according to
   a key definition object. Raises an exception if any of the given key
   doesn't match the key definition. On success, returns a value <0 if
   `key_a` parts are less than `key_b` parts, 0 if equal, >0 if greater.