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/beacon_chain/src/test_utils.rs

@@ -132,8 +132,7 @@ pub fn test_spec<E: EthSpec>() -> ChainSpec {
                 FORK_NAME_ENV_VAR, e
             )
         });
-        let fork = ForkName::from_str(fork_name.as_str())
-            .unwrap_or_else(|()| panic!("unknown FORK_NAME: {}", fork_name));
+        let fork = ForkName::from_str(fork_name.as_str()).unwrap();
         fork.make_genesis_spec(E::default_spec())
     } else {
         E::default_spec()