v2.0: EpochRewards: decode points and rewards fields as Strings (backport of #2501)

[mergify]

Problem

EpochRewards::points can easily overflow the max JSON integer, but we are trying to use serde_json::Number in solana-account-decoder: #760

This results in RPC results that fail JSON parsing, eg:

$ curl -X POST -H "Content-Type: application/json" -d '{"jsonrpc":"2.0", "id":1, "method":"getAccountInfo", "params":["SysvarEpochRewards1111111111111111111111111",{"encoding":"jsonParsed","commitment":"confirmed"}]}' 127.0.0.1:8899 | jq
{
  "jsonrpc": "2.0",
  "result": {
    "context": {
      "apiVersion": "2.0.3",
      "slot": 475138
    },
    "value": {
      "data": [
        "AUAHAAAAAAApAAAAAAAAAIUNPEmDh+dINbalmdonQUN3ihFWtNs3TkzSdK+Iq7bWAAAtdN6RBR8BAAAAAAAAAIYkzjTLAQAA5yWbrzUAAAAB",
        "base64"
      ],
      "executable": false,
      "lamports": 1454640,
      "owner": "Sysvar1111111111111111111111111111111111111",
      "rentEpoch": 0,
      "space": 81
    }
  },
  "id": 1
}

Summary of Changes

Decode points, distributedRewards, and totalRewards as strings
numPartitions should never overflow as it is capped at 43,200
startingBlockHeight could overflow some day, but there are lots of other decoders passing Slot as a number, so this is a more systemic problem and can/should be addressed in one fell swoop.

---

This is an automatic backport of pull request #2501 done by [Mergify](https://mergify.com).

[CriesofCarrots]

Proposing a backport because this is a bug in the EpochRewards sysvar account decoder, which will be instantiated in v2.0. But it's also very low risk, as it only affects account data decoding for RPC nodes.

[behzadnouri]

Amazing, so @anza-xyz/backport-reviewers can approve their own backports while our work is indefinitely blocked. This is great.