Use proto generated struct for Executions

[NicolasMahe]

The Engine should use directly the struct generated by protobuf instead of the current by-hand struct.

To do:

• replace the current by-hand struct by the generated one (make protoc generates in the right folder)
• add hash and validation tags (gogoproto.moretags):
  

  engine/protobuf/types/service.proto

  Lines 16 to 18 in 7f4f7dd

  	string key = 4 [
  	(gogoproto.moretags) = 'hash:"name:1" validate:"printascii"'
  	];

• set gogoproto.customtype for hashes to hash.Hash:
  

  engine/protobuf/types/service.proto

  Lines 180 to 184 in 7f4f7dd

  	bytes hash = 10 [
  	(gogoproto.moretags) = 'validate:"required"',
  	(gogoproto.customtype) = "github.com/mesg-foundation/engine/hash.Hash",
  	(gogoproto.nullable) = false
  	];

• remove mapping function
• update codebase