Hardcode DPNS data contract in the code

[pshenmic]

Issue

DPNS data contract schema previously was being loaded from the filesystem (as it was during development), and I forgot to to put in inside the build. Users who tried to use the binary from the releases had to have a local dpns data contract schema file, or they encounter a error:

thread 'main' panicked at src/commands/register_dpns_name.rs:86:80:
Unable to read file: Os { code: 2, kind: NotFound, message: "No such file or directory" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace 

In this PR I moved schema straight in the code so it is shipped with the binaries

Things done

• Created a constants module with raw DPNS data contract schema