Fixed large files issue, and added support for multi-file uploads

[victaphu]

Fix for harmony-one/explorer-v2-frontend#115, and added support for harmony-one/contract-verification-service#7

Verification for large files

• Added multi-file support so users can upload multiple files in place of one very large file
• Expanded file size limit to 1 MB (previously 100 KB default)
• Etherscan-API

Added support for etherscan api

• Modified hardhat release to support new contract-verification-service API
• Added documentation in gitbook (for review and release)

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