CLI: Bounty management commands (bounties related)

[zeeshanakram3]

This issue outlines that subset of commands for Bounty module based on the extrinsics, for the complete set of commands refer to #3056

• Getting information of a bounty
• List of existing bounties (bounties)
• Creating a bounty (create_bounty)
• Canceling a bounty (cancel_bounty)
• Vetoing a bounty (veto_bounty)

Furthermore, my initial idea for command structure, based on the existing implementation of CLI for different modules, is as follows:

• joystream-cli bounty:bounty
• joystream-cli bounty:bounties
• joystream-cli bounty:createBounty
• joystream-cli bounty:cancelBounty
• joystream-cli bounty:vetoBounty

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