how to check `PAYMENT_PLUGIN` is AINPayment contract

[orth0gonal]

In my first glance, I was thinking the way by checking paymentPlugin_.ainft value is same with
this contract's address. just value comparision. that's all.

Originally posted by @minho-comcom-ai in #3 (comment)

[orth0gonal]

As @minho-comcom-ai said, checking the value of PAYMENT_PLUGIN.ainft is same as this.address(AINFT721) is needed.

However, to compare the values, it seems to be overhead that change type of PAYMENT_PLUGIN from address to AINPayment to ONLY get ainft address. Is there any better option to check this values? If not, I will change this type to AINPayment as @minho-comcom-ai said.

[minho-comcom-ai]

Aha, changing type from address to AINPayment could be necessary for accessing the internal values. Hmm.. let me give a try.