Ref: redis#12760

enabled by default) with older Redis cluster (< 7.0 - extensions not
handled) .

With some of the extensions enabled by default in 7.2 version, new nodes
running 7.2 and above start sending out larger clusterbus message
payload including the ping extensions. This caused an incompatibility
with node running engine versions < 7.0. Old nodes (< 7.0) would receive
the payload from new nodes (> 7.2) would observe a payload length
(totlen) > (estlen) and would perform an early exit and won't process
the message.

This fix introduces a flag `extensions_supported` on the clusterMsg
indicating the sender node can handle extensions parsing. Once, a
receiver nodes receives a message with this flag set to 1, it would
update clusterNode new field extensions_supported and start sending out
extensions if it has any.

This PR also introduces a DEBUG sub command to enable/disable cluster
message extensions `process-clustermsg-extensions` feature.

Note: A successful `PING`/`PONG` is required as a sender for a given
node to be marked as `extensions_supported` and then extensions message
will be sent to it. This could cause a slight delay in receiving the
extensions message(s).

TCL test verifying the cluster state is healthy irrespective of
enabling/disabling cluster message extensions feature.

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Signed-off-by: Harkrishn Patro <harkrisp@amazon.com>