/eth/v1/node/syncing returns wrong types

[timothysu]

Describe the bug
This breaks several external integrations. The following is returned when that endpoint is hit:

{"data":{"head_slot":3343898,"sync_distance":0,"is_syncing":false}}

with the json types of

{
  head_slot: number
  sync_distance: number
  is_syncing: boolean
}

Expected behavior
Per https://ethereum.github.io/beacon-APIs/#/Node/getSyncingStatus , this is supposed to return a response of the following schema:

{
  head_slot: string
  sync_distance: string
  is_syncing: boolean
}

so the proper response should be

{"data":{"head_slot":"3343898","sync_distance":"0","is_syncing":false}}

Steps to Reproduce

curl http://<lodestar-beacon-node>:9596/eth/v1/node/syncing

Screenshots
n/a

Desktop (please complete the following information):

• OS: Ubuntu 20.04 LTS
• Version: chainsafe/lodestar:v0.34.1 via docker
• Branch: n/a
• Commit hash: n/a

[dapplion]

@dadepo Can you take a look please?