Introduces query for contract build information

[jstuczyn]

This pull request solves the problem of "what version of the contract have we exactly deployed?". It introduces a query that returns the build information. Currently it includes the following:

MixnetContractVersion {
    build_timestamp: "2021-11-25T12:18:41.573392838+00:00",
    build_version: "0.1.0",
    commit_sha: "39c7b131b69a8d3217372853afb7e776207b6d19",
    commit_timestamp: "2021-11-25T10:27:31+00:00",
    commit_branch: "feature/contract-version",
    rustc_version: "1.56.1",
}

If anyone feels like something else could be added there, full list of available options is here: https://docs.rs/vergen/5.1.17/vergen/