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.

validator_client/src/http_api/mod.rs

@@ -217,8 +217,7 @@ pub fn serve<T: 'static + SlotClock + Clone, E: EthSpec>(
         .and(signer.clone())
         .and_then(|spec: Arc<_>, signer| {
             blocking_signed_json_task(signer, move || {
-                let mut config = ConfigAndPreset::from_chain_spec::<E>(&spec);
-                config.make_backwards_compat(&spec);
+                let config = ConfigAndPreset::from_chain_spec::<E>(&spec, None);
                 Ok(api_types::GenericResponse::from(config))
             })
         });