Expose version metadata

[Levi0804]

Closes #77

This PR is a work in progress. Please let me know if the current approach looks good.

In the current approach I have created a VERSION_METADATA env variable (from build.rs) which holds a stringified JSON of our version metadata. This JSON string could then be deserialized appropriately and served at the correct endpoint.

Current structure of JSON:

{
  "versions": [
    { "name": "iroha", "version": "2.0.0-rc.2.0" },
    { "name": "iroha_explorer", "version": "0.2.1" },
    { "name": "iroha_config", "version": "2.0.0-rc.2.0" },
    { "name": "iroha_config_base", "version": "2.0.0-rc.2.0" }
    // ...
  ]
}

[0x009922]

@Levi0804 Hi, thanks for opening the PR!

I think it is too excessive to handle all versions of Iroha dependencies. All Iroha deps are of the same version anyway.

What matters is:

• Show which Explorer version it is (what is in Cargo.toml)
• Show which Iroha version it is compatible with. I'd extract it as version of iroha_data_model dependency acquired from cargo metadata

So, the result would be just 2 env vars to pass from build.rs - VERGEN_EXPLORER_VERSION and VERGEN_IROHA_COMPAT or something.

How does it sound?

[Levi0804]

@Levi0804 Hi, thanks for opening the PR!

Happy to contribute!

• Show which Explorer version it is (what is in Cargo.toml)

We could also get this from cargo metadata, should I just get it from there?

How does it sound?

Sounds good to me!

Do you think I should switch to using the vergen crate, or is this implementation okay?

[0x009922]

We could also get this from cargo metadata, should I just get it from there?

Yes.

Do you think I should switch to using the vergen crate, or is this implementation okay?

No need to use the vergen crate (I used VERGEN_ prefix because it reflects the essence of it). I also removed vergen in another unfinished PR: #72

[0x009922]

LGTM, only a small naming change