Add cw721-receiver contract

[chipshort]

This implements an example contract that can receive a cw721 token and then just succeed or error depending on the passed message.

Also fine if you don't want to merge. We just needed this for a wasmd test. In that case, just close it.

[JakeHartnell]

This is really helpful! Have had to make similar contracts to this myself... maybe we need a test-contracts folder? Open to ideas from others but would be down to have some test contracts like this.

[Art3miX]

This is really helpful! Have had to make similar contracts to this myself... maybe we need a test-contracts folder? Open to ideas from others but would be down to have some test contracts like this.

I think having a test-contracts folder is smart, you almost always need a test contract if you work with cw721

[Art3miX]

Cute little contract.

P.s, need CI fix.

[chipshort]

I applied @Art3miX's feedback.
Should I create a test-contracts folder and move it there?

[Art3miX]

LGTM

[taitruong]

I applied @Art3miX's feedback. Should I create a test-contracts folder and move it there?

Just rename it to cw721-receiver-tester, no extra folder needed imo. Once done we can merge it then.

[chipshort]

Can someone merge? I don't have write access here

[JakeHartnell]

Amazing! This will be really useful.