feat(prose/golang): accept purpose of use via config for now

For some reason peer metadata is not included with each request, so we cannot
reliably extract purpose of use from there. Before we figure it out, we are
going to accept it as a configuration option.

privacy-profile-composer/pkg/envoyfilter/config.go

@@ -18,6 +18,7 @@ type config struct {
 	opaConfig     string
 	presidioUrl   string
 	internalCidrs []net.IPNet
+	purpose       string
 }
 
 type ConfigParser struct {
@@ -104,6 +105,14 @@ func (p *ConfigParser) Parse(any *anypb.Any, callbacks api.ConfigCallbackHandler
 		}
 	}
 
+	if purpose, ok := configStruct["purpose"]; !ok {
+		return nil, fmt.Errorf("missing purpose")
+	} else if str, ok := purpose.(string); !ok {
+		return nil, fmt.Errorf("purpose: expect string while got %T", purpose)
+	} else {
+		conf.purpose = str
+	}
+
 	return conf, nil
 }
 
@@ -130,6 +139,7 @@ func (p *ConfigParser) Merge(parent interface{}, child interface{}) interface{}
 
 	newConfig.opaEnforce = childConfig.opaEnforce
 	newConfig.internalCidrs = childConfig.internalCidrs
+	newConfig.purpose = childConfig.purpose
 
 	return &newConfig
 }

privacy-profile-composer/pkg/envoyfilter/filter.go

@@ -312,7 +312,7 @@ func (f *Filter) runOPA(ctx context.Context, isDecode bool, dataItems []string)
 			//  as simple.rego expects PII type & purpose to be passed as headers
 			//  (i.e. as if we had an OPA sidecar)
 			Input: map[string]interface{}{
-				"purpose_of_use": f.headerMetadata.Purpose,
+				"purpose_of_use": f.config.purpose,
 				"data_items":     dataItems,
 				// todo double check that this is non-null only in outbound and decode mode
 				"external_domain": f.thirdPartyURL, // path or null