Implemented IWallet interface

[sirano11]

Proposed changes

This PR has the following contents.

• Defined the IWallet interface and implemented it to the KeyringContainer class.
• To use IWallet interface in Contract class, modify return type Caver@getWallet()
  • also when signing in Contract class, it modified code to caver.getWallet().sign()

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Further comments

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[jimni1222]

LGTM except for minor comment :)