fix(telemetry): make the consensus trace strategy an enum

consensus_trace_strategy was read as a std::string and compared against the
literal "attribute" in startRoundTracing(), while the runbook documented
"deterministic" and "random". The documented value "random" therefore fell
through to the default and did nothing.

Parse the setting once into ConsensusTraceStrategy, so the consensus code
branches on a type. The accepted spellings are now "deterministic" and
"random"; anything else fails at startup instead of silently defaulting.
The behaviour behind the old "attribute" name is unchanged and is now
reached by "random".

Document consensus_trace_strategy in xrpld-example.cfg, stating that
"random" is experimental and not used: it gives each node its own trace id,
so one round arrives as one trace per node.

Also state on the tx.included event that it covers the agreed consensus set
before the ledger is built, so it is a superset of the accepted ledger.

src/libxrpl/telemetry/Telemetry.cpp

@@ -216,7 +216,7 @@ public:
         return false;
     }
 
-    [[nodiscard]] std::string const&
+    [[nodiscard]] ConsensusTraceStrategy
     getConsensusTraceStrategy() const override
     {
         return setup_.consensusTraceStrategy;
@@ -436,7 +436,7 @@ public:
         return setup_.traceLedger;
     }
 
-    [[nodiscard]] std::string const&
+    [[nodiscard]] ConsensusTraceStrategy
     getConsensusTraceStrategy() const override
     {
         return setup_.consensusTraceStrategy;