StandartHookMetadata construction fix

[EgeCaner]

StandartHookMetadata contstruction fix.

• Variant const to u8 to u16.
• Variant added to metadata with append_u16 instead of casting to u128 which results in error due trying to read it via read_u16 in StandartHookMetadata::variant.
• u256 variables appended via append_u256
• Zero address check for required_hook in mailbox ctor
• Zero address check for setters for required hook & default hook & default ism in mailbox
• Zero address handling for custom hook in mailbox-dispatch alongside Option::None