use protojson & default instance_label to policy_name

[gertd]

The aserto dl stream command uses JSON marshal instead of protojson resulting in:

"timestamp": {
    "seconds": 1723586632,
    "nanos": 713391185
  }

instead of:

"timestamp":  "2024-08-13T23:14:18.752872404Z"

enums as numbers instead of enum names

    "context": {
      "identity": "[email protected]",
      "type": 2
    },

instead of:

"context":  {
      "identity":  "[email protected]",
      "type":  "IDENTITY_TYPE_SUB"
    }

When instance_label is not provided, the policy_name is used.

if cmd.InstanceLabel == "" && cmd.PolicyName != "" {
		cmd.InstanceLabel = cmd.PolicyName
	}