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/src/config.rs

@@ -116,7 +116,14 @@ pub fn get_config<E: EthSpec>(
     }
 
     if cli_args.is_present("http-disable-legacy-spec") {
-        client_config.http_api.serve_legacy_spec = false;
+        warn!(
+            log,
+            "The flag --http-disable-legacy-spec is deprecated and will be removed"
+        );
+    }
+
+    if let Some(fork_name) = clap_utils::parse_optional(cli_args, "http-spec-fork")? {
+        client_config.http_api.spec_fork_name = Some(fork_name);
     }
 
     if cli_args.is_present("http-enable-tls") {