add attestation inclusion distance in http api (#4148)
## Issue Addressed
#4097
## Proposed Changes
Add attestation inclusion distance in http api, extend `/lighthouse/ui/validator_metrics` to include it.
## Usage
```
curl -X POST "http://localhost:8001/lighthouse/ui/validator_metrics" -d '{"indices": [1]}' -H "Content-Type: application/json" | jq
```
```
{
"data": {
"validators": {
"1": {
"attestation_hits": 3,
"attestation_misses": 1,
"attestation_hit_percentage": 75,
"attestation_head_hits": 3,
"attestation_head_misses": 0,
"attestation_head_hit_percentage": 100,
"attestation_target_hits": 3,
"attestation_target_misses": 0,
"attestation_target_hit_percentage": 100,
"attestation_inclusion_distance": 1
}
}
}
}
```
## Additional Info
NA
90d562b3d4