Serve Bellatrix preset in BN API (#3425)

## Issue Addressed

Resolves #3388
Resolves #2638

## Proposed Changes

- Return the `BellatrixPreset` on `/eth/v1/config/spec` by default.
- Allow users to opt out of this by providing `--http-spec-fork=altair` (unless there's a Bellatrix fork epoch set).
- Add the Altair constants from #2638 and make serving the constants non-optional (the `http-disable-legacy-spec` flag is deprecated).
- Modify the VC to only read the `Config` and not to log extra fields. This prevents it from having to muck around parsing the `ConfigAndPreset` fields it doesn't need.

## Additional Info

This change is backwards-compatible for the VC and the BN, but is marked as a breaking change for the removal of `--http-disable-legacy-spec`.

I tried making `Config` a `superstruct` too, but getting the automatic decoding to work was a huge pain and was going to require a lot of hacks, so I gave up in favour of keeping the default-based approach we have now.

beacon_node/http_api/src/lib.rs

@@ -104,9 +104,9 @@ pub struct Config {
     pub listen_addr: IpAddr,
     pub listen_port: u16,
     pub allow_origin: Option<String>,
-    pub serve_legacy_spec: bool,
     pub tls_config: Option<TlsConfig>,
     pub allow_sync_stalled: bool,
+    pub spec_fork_name: Option<ForkName>,
 }
 
 impl Default for Config {
@@ -116,9 +116,9 @@ impl Default for Config {
             listen_addr: IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)),
             listen_port: 5052,
             allow_origin: None,
-            serve_legacy_spec: true,
             tls_config: None,
             allow_sync_stalled: false,
+            spec_fork_name: None,
         }
     }
 }
@@ -1534,18 +1534,15 @@ pub fn serve<T: BeaconChainTypes>(
         });
 
     // GET config/spec
-    let serve_legacy_spec = ctx.config.serve_legacy_spec;
+    let spec_fork_name = ctx.config.spec_fork_name;
     let get_config_spec = config_path
         .and(warp::path("spec"))
         .and(warp::path::end())
         .and(chain_filter.clone())
         .and_then(move |chain: Arc<BeaconChain<T>>| {
             blocking_json_task(move || {
-                let mut config_and_preset =
-                    ConfigAndPreset::from_chain_spec::<T::EthSpec>(&chain.spec);
-                if serve_legacy_spec {
-                    config_and_preset.make_backwards_compat(&chain.spec);
-                }
+                let config_and_preset =
+                    ConfigAndPreset::from_chain_spec::<T::EthSpec>(&chain.spec, spec_fork_name);
                 Ok(api_types::GenericResponse::from(config_and_preset))
             })
         });