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/lib.rs

@@ -977,7 +977,9 @@ impl BeaconNodeHttpClient {
     }
 
     /// `GET config/spec`
-    pub async fn get_config_spec(&self) -> Result<GenericResponse<ConfigAndPreset>, Error> {
+    pub async fn get_config_spec<T: Serialize + DeserializeOwned>(
+        &self,
+    ) -> Result<GenericResponse<T>, Error> {
         let mut path = self.eth_path(V1)?;
 
         path.path_segments_mut()

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()

common/eth2/src/mixin.rs

@@ -21,17 +21,17 @@ impl ResponseOptional for Result<Response, Error> {
 /// Trait for extracting the fork name from the headers of a response.
 pub trait ResponseForkName {
     #[allow(clippy::result_unit_err)]
-    fn fork_name_from_header(&self) -> Result<Option<ForkName>, ()>;
+    fn fork_name_from_header(&self) -> Result<Option<ForkName>, String>;
 }
 
 impl ResponseForkName for Response {
-    fn fork_name_from_header(&self) -> Result<Option<ForkName>, ()> {
+    fn fork_name_from_header(&self) -> Result<Option<ForkName>, String> {
         self.headers()
             .get(CONSENSUS_VERSION_HEADER)
             .map(|fork_name| {
                 fork_name
                     .to_str()
-                    .map_err(|_| ())
+                    .map_err(|e| e.to_string())
                     .and_then(ForkName::from_str)
             })
             .transpose()