Add an auction functionality to the escrow module

[LeCodeurDuDimanche]

Add auction functionality to the existing escrow module

[LeCodeurDuDimanche]

• Implement the data types and basic verification
• Determine the behavior of an auction when it is expired (automatic completion ?)
• Should we allow modifying the seller of an auction after it was created ?
• Give a specific fee for creating an auction
• Give a specific fee for bids (notably for the broker commission)
• Can a bidder add other tokens to the price ?
• Update the keeper's methods
• Add isAuction field as secondary key for searching/filtering data
• Do the auction completion logic
• Update the CLI/REST interfaces
• Add tests and update invariants

[LeCodeurDuDimanche]

• Add the gRPC Service to complete an auction