rpc server: add `health/readiness endpoint` (#4802)

Previous attempt https://github.com/paritytech/substrate/pull/14314

Close #4443 

Ideally, we should move /health and /health/readiness to the prometheus
server but because it's was quite easy to implement on the RPC server
and that RPC server already exposes /health.

Manual tests on a polkadot node syncing:

```bash
➜ polkadot-sdk (na-fix-4443) ✗ curl -v localhost:9944/health
* Host localhost:9944 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:9944...
* connect to ::1 port 9944 from ::1 port 55024 failed: Connection refused
*   Trying 127.0.0.1:9944...
* Connected to localhost (127.0.0.1) port 9944
> GET /health HTTP/1.1
> Host: localhost:9944
> User-Agent: curl/8.5.0
> Accept: */*
>
< HTTP/1.1 200 OK
< content-type: application/json; charset=utf-8
< content-length: 53
< date: Fri, 14 Jun 2024 16:12:23 GMT
<
* Connection #0 to host localhost left intact
{"peers":0,"isSyncing":false,"shouldHavePeers":false}%
➜ polkadot-sdk (na-fix-4443) ✗ curl -v localhost:9944/health/readiness
* Host localhost:9944 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:9944...
* connect to ::1 port 9944 from ::1 port 54328 failed: Connection refused
*   Trying 127.0.0.1:9944...
* Connected to localhost (127.0.0.1) port 9944
> GET /health/readiness HTTP/1.1
> Host: localhost:9944
> User-Agent: curl/8.5.0
> Accept: */*
>
< HTTP/1.1 500 Internal Server Error
< content-type: application/json; charset=utf-8
< content-length: 0
< date: Fri, 14 Jun 2024 16:12:36 GMT
<
* Connection #0 to host localhost left intact
```

//cc @BulatSaif

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---------

Co-authored-by: Bastian Köcher <[email protected]>