Fix NonFungibleTokenApprovalReceiver::nft_on_approve return type

[agostbiro]

Problem

The NonFungibleTokenApprovalReceiver::nft_on_approve trait method currently has a near_sdk::PromiseOrValue<String> return type, but the standard mandates a void return type as reported in #693.

Possible Solutions

The void return type in the standard is probably an oversight, since in the documentation of the nft_approve interface it says that it "returns promise call to nft_on_approve, which can resolve with whatever it wants." This suggests that the intended return type nft_on_approve in the standard was any. We could submit an errata to fix this.

Question is how to represent the any return type of NonFungibleTokenApprovalReceiver::nft_on_approve in Rust? I see the following options:

1. The obvious solution would be to use Box<dyn std::any::Any>, but this won't work, because the trait Serialize is not implemented for dyn Any.
2. An associated type on the trait (this might not play nicely with bindgen macros) (h/t @austinabell )
3. Require the implementation to serialize the value. We could express this in the type system by having NonFungibleTokenApprovalReceiver::nft_on_approve return near_sdk::PromiseOrValue<serde_json::Value>. This would be a good solution, but we need to support borsh as well. I'm not too familiar with borsh, but as far as I can tell, borsh::schema::BorshSchema achieves the same result, so we could have NonFungibleTokenApprovalReceiver::nft_on_approve return an enum value that wraps either serde_json::Value or BorshSchema, but this might be getting too complex.
4. Leave the return type near_sdk::PromiseOrValue<String> since anything can be serialized into a string and it hasn't caused any problems so far except for some confusion around the documentation.

Edit: added associate type solution from Austin originally written in #918.