Lighthouse v0.2.0 (Medalla) (#1452)

## Issue Addressed

NA

## Proposed Changes

- Moves the git-based versioning we were doing into the `lighthouse_version` crate in `common`.
- Removes the `beacon_node/version` crate, replacing it with `lighthouse_version`.
- Bumps the version to `v0.2.0`.

## Additional Info

There are now two types of version string:

1. `const VERSION: &str = Lighthouse/v0.2.0-1419501f2+`
1. `version_with_platform() = Lighthouse/v0.2.0-1419501f2+/x86_64-linux`

(1) is handy cause it's a `const` and shorter. (2) has platform info so it's more useful. Note that the plus-sign (`+`) indicates the the git commit is dirty (it used to be `(modified)` but I had to shorten it to fit into graffiti).

These version strings are now included on:

- `lighthouse --version`
- `lcli --version`
- `curl localhost:5052/node/version`
- p2p messages when we communicate our version

You can update the version by changing this constant (version is not related to a `Cargo.toml`):

b9ad7102d5/common/lighthouse_version/src/lib.rs (L4-L15)

beacon_node/rest_api/Cargo.toml

@@ -12,7 +12,6 @@ beacon_chain = { path = "../beacon_chain" }
 network = { path = "../network" }
 eth2_libp2p = { path = "../eth2_libp2p" }
 store = { path = "../store" }
-version = { path = "../version" }
 serde = { version = "1.0.110", features = ["derive"] }
 serde_json = "1.0.52"
 serde_yaml = "0.8.11"
@@ -40,6 +39,7 @@ environment = { path = "../../lighthouse/environment" }
 uhttp_sse = "0.5.1"
 bus = "2.2.3"
 itertools = "0.9.0"
+lighthouse_version = { path = "../../common/lighthouse_version" }
 
 [dev-dependencies]
 assert_matches = "1.3.0"