Add ADR-043 BaseNFT Module

docs/architecture/adr-043-basenft-module.md

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+# ADR 43: BaseNFT Module
+
+## Changelog
+
+- 05.05.2021: Initial Draft
+
+## Status
+
+DRAFT
+
+## Abstract
+
+This ADR defines a generic NFT module of `x/nft` which supports NFTs as a `proto.Any` and contains `BaseNFT` as the default implementation.
+
+## Context
+
+As was discussed in [#9065](https://github.com/cosmos/cosmos-sdk/discussions/9065), several potential solutions can be considered:
+- irismod/nft and modules/incubator/nft
+- CW721
+- DID NFTs
+- interNFT
+
+Considering generic usage and compatibility of interchain protocols including IBC and Gravity Bridge, it is preferred to have a very simple NFT module design which only stores NFTs by id and owner. 
+
+Application-specific functions (minting, burning, transferring, etc.) should be managed by other modules on Cosmos Hub or on other Zones.
+
+The current design is based on the work done by [IRISnet team](https://github.com/irisnet/irismod/tree/master/modules/nft) and an older implementation in the [Cosmos repository](https://github.com/cosmos/modules/tree/master/incubator/nft).
+
+
+## Decision
+
+We will create a module `x/nft` which only stores NFTs by id and owner.
+
+The interface for the `x/nft` module:
+
+```go
+// NFTI is an interface used to store NFTs at a given id and owner.
+type NFTI interface {
+	GetId() string // can not return empty string.
+	GetOwner() sdk.AccAddress
+}
+```
+
+We will also create `BaseNFT` as the default implementation of the `NFTI` interface:
+```proto
+message BaseNFT {
+  option (gogoproto.equal) = true;
+
+  string id    = 1;
+  string name  = 2;
+  string uri   = 3 [(gogoproto.customname) = "URI"];
+  string data  = 4;
+  string owner = 5;
+}
+```
+
+Queries functions for `BaseNFT` is:
+```proto
+service Query {
+
+  // NFT queries NFT details based on id.
+  rpc NFT(QueryNFTRequest) returns (QueryNFTResponse) {
+    option (google.api.http).get = "/cosmos/nft/v1beta1/nfts/{id}";
+  }
+
+  // NFTs queries all proposals based on the optional onwer.
+  rpc NFTs(QueryNFTsRequest) returns (QueryNFTsResponse) {
+    option (google.api.http).get = "/cosmos/nft/v1beta1/nfts";
+  }
+}
+
+// QueryNFTRequest is the request type for the Query/NFT RPC method
+message QueryNFTRequest {
+  string id = 1;
+}
+
+// QueryNFTResponse is the response type for the Query/NFT RPC method
+message QueryNFTResponse {
+  google.protobuf.Any nft = 1 [(cosmos_proto.accepts_interface) = "NFTI", (gogoproto.customname) = "NFT"];
+}
+
+// QueryNFTsRequest is the request type for the Query/NFTs RPC method
+message QueryNFTsRequest {
+  string                                 owner      = 1;
+  cosmos.base.query.v1beta1.PageResponse pagination = 2;
+}
+
+// QueryNFTsResponse is the response type for the Query/NFTs RPC method
+message QueryNFTsResponse {
+  repeated google.protobuf.Any nfts = 1 [(cosmos_proto.accepts_interface) = "NFTI", (gogoproto.customname) = "NFTs"];
+  cosmos.base.query.v1beta1.PageResponse pagination = 2;
+}
+```
+
+
+
+## Consequences
+
+### Backward Compatibility
+
+No backwards incompatibilities.
+
+### Positive
+
+- NFT functions available on Cosmos Hub
+- NFT module which supports interoperability with IBC and other cross-chain infrastractures like Gravity Bridge
+
+### Negative
+
+### Neutral
+
+- Other functions need more modules. For example, a custody module is needed for NFT trading function, a collectible module is needed for defining NFT properties
+
+## Further Discussions
+
+For other kinds of applications on the Hub, more app-specific modules can be developed in the future:
+- `x/collectibles`: grouping NFTs into collections and defining properties of NFTs such as minting, burning and transferring, etc.
+- `x/nft_custody`: custody of NFTs to support trading functionality
+- `x/nft_marketplace`: selling and buying NFTs using sdk.Coins
+
+Other networks in the Cosmos ecosystem could design and implement their own NFT modules for specific NFT applications and usecases.
+
+## References
+
+- Initial discussion: https://github.com/cosmos/cosmos-sdk/discussions/9065
+- x/nft: initialize module: https://github.com/cosmos/cosmos-sdk/pull/9174