Burn assets by group key

[darioAnongba]

Enable burning of assets by specifying a group key.

• Add AssetSpecifier to tapcommon.proto and use it in the BurnAssetRequest
• Change tapcli to accept group_key param in tapd assets burn commands
• Update tapfreighter to support passing a group key in the asset specifier
• Update BurnAsset RPC request to accept an AssetSpecifier. Breaking change!
• Update integration tests

Example:

• Mint asset in group key X: 1000 units
• Mint asset in group X: 100 units
• Burn 1080 units
• Creates 3 outputs:
  • change: 20 units
  • burn output: 1000 units
  • burn output: 80 units