Missing endpoint in docs / auto-generated client

[janniks]

The data_var endpoint is missing from our docs and the auto-generated client code. https://github.com/stacks-network/stacks-blockchain/blob/master/docs/rpc-endpoints.md#get-v2data_varstacks-addresscontract-namevar-name

Also missing are traits and constant_val.

[zone117x]

Note that we still need to replace the RPC endpoint OpenAPI spec in this repo with this: https://github.com/stacks-network/stacks-blockchain/blob/master/docs/rpc/openapi.yaml

However, this endpoint doesn't seem to be defined there either.