Add new optional fields to Entry type

proto/spire/api/types/entry.proto

@@ -64,6 +64,21 @@ message Entry {
 
     // When the entry was created (seconds since Unix epoch).
     int64 created_at = 15;
+
+    /** This nested message is reserved to contain a number of optional fields
+    controlling the various aspects of the agent's behaviour with respect to a
+    given registration entry. It serves to enable introducing and testing out new
+    tunables, without having to modify the datastore schema. Over time, some of
+    the fields contained therein may be considered eligible for their dedicated
+    attributes in the datastore. */
+    message AdditionalAttributes {
+        /** Flag indicating whether the agent should prefetch and cache X509 SVID.
+        Can be set to `true` if the workload is unlikely to request an X509 SVID.
+        This is meant to prevent unnecessary effort spent on generating SVIDs of types,
+        which are unlikely to be needed.*/
+        bool disable_x509_svid_prefetch = 1;
+    }
+    optional AdditionalAttributes additional_attributes = 16;
 }
 
 // Field mask for Entry fields
@@ -109,4 +124,7 @@ message EntryMask {
 
     // created_at field mask
     bool created_at = 15;
+
+    // additional_attributes field mask
+    bool additional_attributes = 16;
 }