Add overloading support in MariaDB_FUNCTION_PLUGIN

Under terms of MDEV-20837 we added MariaDB_FUNCTION_PLUGIN.

Every function plugin exports a Create_func object. This is the traditional way of defining functions in MySQL/MariaDB. But it has a disadvantage: it does not support function overloading.

So two plugin authors cannot create functions with the same name but different signatures, e.g.

SOME_FUNC(geometry);

SOME_FUNC(inet6);

Inside the server we already have Item_handled_func, which supports overloading through its Item_handled_func::Handler, and which is used by some functions:

• Bit operators | & ^ << >> ~
• bit_expr + INTERVAL
• BIT_COUNT()
• ADDTIME()
• STR_TO_DATE()
• CAST(expr AS CHAR)

For example, ADDTIME() has the following overloads:

ADDTIME(DATE or DATETIME or TIMESTAMP, any scalar type)

ADDTIME(TIME, any scalar type)

ADDTIME(Any string type, any scalar type)

Let's expose this interface to function plugins, so a function plugin exports:

• either a traditional style Create_func object (without overloading support)
• or a handler for Item_handled_func (with overloading support)