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/beacon_node_fallback.rs

@@ -7,7 +7,7 @@ use crate::http_metrics::metrics::{inc_counter_vec, ENDPOINT_ERRORS, ENDPOINT_RE
 use environment::RuntimeContext;
 use eth2::BeaconNodeHttpClient;
 use futures::future;
-use slog::{debug, error, info, warn, Logger};
+use slog::{error, info, warn, Logger};
 use slot_clock::SlotClock;
 use std::fmt;
 use std::fmt::Debug;
@@ -16,7 +16,7 @@ use std::marker::PhantomData;
 use std::sync::Arc;
 use std::time::Duration;
 use tokio::{sync::RwLock, time::sleep};
-use types::{ChainSpec, EthSpec};
+use types::{ChainSpec, Config, EthSpec};
 
 /// The number of seconds *prior* to slot start that we will try and update the state of fallback
 /// nodes.
@@ -213,9 +213,9 @@ impl<E: EthSpec> CandidateBeaconNode<E> {
 
     /// Checks if the node has the correct specification.
     async fn is_compatible(&self, spec: &ChainSpec, log: &Logger) -> Result<(), CandidateError> {
-        let config_and_preset = self
+        let config = self
             .beacon_node
-            .get_config_spec()
+            .get_config_spec::<Config>()
             .await
             .map_err(|e| {
                 error!(
@@ -228,25 +228,15 @@ impl<E: EthSpec> CandidateBeaconNode<E> {
             })?
             .data;
 
-        let beacon_node_spec =
-            ChainSpec::from_config::<E>(&config_and_preset.config).ok_or_else(|| {
-                error!(
-                    log,
-                    "The minimal/mainnet spec type of the beacon node does not match the validator \
-                    client. See the --network command.";
-                    "endpoint" => %self.beacon_node,
-                );
-                CandidateError::Incompatible
-            })?;
-
-        if !config_and_preset.extra_fields.is_empty() {
-            debug!(
+        let beacon_node_spec = ChainSpec::from_config::<E>(&config).ok_or_else(|| {
+            error!(
                 log,
-                "Beacon spec includes unknown fields";
+                "The minimal/mainnet spec type of the beacon node does not match the validator \
+                client. See the --network command.";
                 "endpoint" => %self.beacon_node,
-                "fields" => ?config_and_preset.extra_fields,
             );
-        }
+            CandidateError::Incompatible
+        })?;
 
         if beacon_node_spec.genesis_fork_version != spec.genesis_fork_version {
             error!(