Weak subjectivity start from genesis (#1675)

## Issue Addressed
Solution 2 proposed here: https://github.com/sigp/lighthouse/issues/1435#issuecomment-692317639

## Proposed Changes
- Adds an optional `--wss-checkpoint` flag that takes a string `root:epoch`
- Verify that the given checkpoint exists in the chain, or that the the chain syncs through this checkpoint. If not, shutdown and prompt the user to purge state before restarting.

## Additional Info


Co-authored-by: Paul Hauner <paul@paulhauner.com>

beacon_node/beacon_chain/src/chain_config.rs

@@ -1,4 +1,5 @@
 use serde_derive::{Deserialize, Serialize};
+use types::Checkpoint;
 
 /// There is a 693 block skip in the current canonical Medalla chain, we use 700 to be safe.
 pub const DEFAULT_IMPORT_BLOCK_MAX_SKIP_SLOTS: u64 = 700;
@@ -10,12 +11,17 @@ pub struct ChainConfig {
     ///
     /// If `None`, there is no limit.
     pub import_max_skip_slots: Option<u64>,
+    /// A user-input `Checkpoint` that must exist in the beacon chain's sync path.
+    ///
+    /// If `None`, there is no weak subjectivity verification.
+    pub weak_subjectivity_checkpoint: Option<Checkpoint>,
 }
 
 impl Default for ChainConfig {
     fn default() -> Self {
         Self {
             import_max_skip_slots: Some(DEFAULT_IMPORT_BLOCK_MAX_SKIP_SLOTS),
+            weak_subjectivity_checkpoint: None,
         }
     }
 }