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/tests/common.rs

@@ -141,9 +141,9 @@ pub async fn create_api_server_on_port<T: BeaconChainTypes>(
             listen_addr: IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1)),
             listen_port: port,
             allow_origin: None,
-            serve_legacy_spec: true,
             tls_config: None,
             allow_sync_stalled: false,
+            spec_fork_name: None,
         },
         chain: Some(chain.clone()),
         network_tx: Some(network_tx),