Add API to compute discrete validator attestation performance (#2874)

## Issue Addressed

N/A

## Proposed Changes

Add a HTTP API which can be used to compute the attestation performances of a validator (or all validators) over a discrete range of epochs.
Performances can be computed for a single validator, or for the global validator set. 

## Usage
### Request
The API can be used as follows:
```
curl "http://localhost:5052/lighthouse/analysis/attestation_performance/{validator_index}?start_epoch=57730&end_epoch=57732"
```
Alternatively, to compute performances for the global validator set:
```
curl "http://localhost:5052/lighthouse/analysis/attestation_performance/global?start_epoch=57730&end_epoch=57732"
```

### Response
The response is JSON formatted as follows:
```
[
  {
    "index": 72,
    "epochs": {
      "57730": {
        "active": true,
        "head": false,
        "target": false,
        "source": false
      },
      "57731": {
        "active": true,
        "head": true,
        "target": true,
        "source": true,
        "delay": 1
      },
      "57732": {
        "active": true,
        "head": true,
        "target": true,
        "source": true,
        "delay": 1
      },
    }
  }
]
```
> Note that the `"epochs"` are not guaranteed to be in ascending order. 

## Additional Info

- This API is intended to be used in our upcoming validator analysis tooling (#2873) and will likely not be very useful for regular users. Some advanced users or block explorers may find this API useful however.
- The request range is limited to 100 epochs (since the range is inclusive and it also computes the `end_epoch` it's actually 101 epochs) to prevent Lighthouse using exceptionally large amounts of memory.

common/eth2/src/lighthouse.rs

@@ -1,5 +1,6 @@
 //! This module contains endpoints that are non-standard and only available on Lighthouse servers.
 
+mod attestation_performance;
 mod block_rewards;
 
 use crate::{
@@ -14,6 +15,9 @@ use ssz::four_byte_option_impl;
 use ssz_derive::{Decode, Encode};
 use store::{AnchorInfo, Split};
 
+pub use attestation_performance::{
+    AttestationPerformance, AttestationPerformanceQuery, AttestationPerformanceStatistics,
+};
 pub use block_rewards::{AttestationRewards, BlockReward, BlockRewardMeta, BlockRewardsQuery};
 pub use lighthouse_network::{types::SyncState, PeerInfo};
 

common/eth2/src/lighthouse/attestation_performance.rs

@@ -0,0 +1,39 @@
+use serde::{Deserialize, Serialize};
+use std::collections::HashMap;
+use types::Epoch;
+
+#[derive(Debug, Default, PartialEq, Clone, Serialize, Deserialize)]
+pub struct AttestationPerformanceStatistics {
+    pub active: bool,
+    pub head: bool,
+    pub target: bool,
+    pub source: bool,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub delay: Option<u64>,
+}
+
+#[derive(Debug, Default, PartialEq, Clone, Serialize, Deserialize)]
+pub struct AttestationPerformance {
+    pub index: u64,
+    pub epochs: HashMap<u64, AttestationPerformanceStatistics>,
+}
+
+impl AttestationPerformance {
+    pub fn initialize(indices: Vec<u64>) -> Vec<Self> {
+        let mut vec = Vec::with_capacity(indices.len());
+        for index in indices {
+            vec.push(Self {
+                index,
+                ..Default::default()
+            })
+        }
+        vec
+    }
+}
+
+/// Query parameters for the `/lighthouse/analysis/attestation_performance` endpoint.
+#[derive(Debug, PartialEq, Clone, Serialize, Deserialize)]
+pub struct AttestationPerformanceQuery {
+    pub start_epoch: Epoch,
+    pub end_epoch: Epoch,
+}