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.

common/eth2/src/lighthouse_vc/http_client.rs

@@ -354,7 +354,9 @@ impl ValidatorClientHttpClient {
     }
 
     /// `GET lighthouse/spec`
-    pub async fn get_lighthouse_spec(&self) -> Result<GenericResponse<ConfigAndPreset>, Error> {
+    pub async fn get_lighthouse_spec<T: Serialize + DeserializeOwned>(
+        &self,
+    ) -> Result<GenericResponse<T>, Error> {
         let mut path = self.server.full.clone();
 
         path.path_segments_mut()