entc: support schema/blob annotation

schema/field/field.go

@@ -660,6 +660,13 @@ func (b *bytesBuilder) Optional() *bytesBuilder {
 	return b
 }
 
+// Blob indicates that this field stores its data in external blob storage
+// rather than in the database. Blob fields have no SQL column.
+func (b *bytesBuilder) Blob() *bytesBuilder {
+	b.desc.Blob = true
+	return b
+}
+
 // Sensitive fields not printable and not serializable.
 func (b *bytesBuilder) Sensitive() *bytesBuilder {
 	b.desc.Sensitive = true
@@ -1439,6 +1446,7 @@ type Descriptor struct {
 	Comment          string                  // field comment.
 	Deprecated       bool                    // mark the field as deprecated.
 	DeprecatedReason string                  // deprecation reason.
+	Blob             bool                    // field is stored in external blob storage.
 	Err              error
 }