feat: add default message tag (#6355)

Description
---
Add a default message tag to `FinalSendMessageParams`. This enables the
tracing of ping-pong and other messages across nodes.

Motivation and Context
---
- We have a related issue with ping-pong, see #6356. This tracing will
assist in analysing the issue.
- With transaction testing, we noticed that some messages have a zero
tag; this will also help that tracing.

How Has This Been Tested?
---
System-level tests.

What process can a PR reviewer use to test or verify this change?
---
Review code.

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Breaking Changes
---

- [x] None
- [ ] Requires data directory on base node to be deleted
- [ ] Requires hard fork
- [ ] Other - Please specify

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comms/dht/src/outbound/message_params.rs

@@ -84,7 +84,7 @@ impl Default for FinalSendMessageParams {
             is_discovery_enabled: false,
             dht_header: None,
             debug_info: None,
-            tag: None,
+            tag: Some(MessageTag::new()),
         }
     }
 }