Add getRewards API in rpc

[nohkwak]

Proposed changes

A new JSON-RPC method provides historic reward distribution details.
This PR adds a new API 'getRewards' in caver-js to call JSON-RPC method 'klay_getRewards'.

Types of changes

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• Bugfix
• New feature or enhancement
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Checklist

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Related issues

• https://github.com/klaytn/kips/blob/master/KIPs/kip-82.md

Further comments

• N/A

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[nohkwak]

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