Refactor cluster backend to introduce NamingBackend trait and impleme…

[reiase]

…nt GossipBackend and HeadNodeBackend

• Replaced GossipCluster with a new NamingBackend trait for improved abstraction in cluster management.
• Added GossipBackend to wrap existing GossipCluster functionality and implement NamingBackend.
• Introduced HeadNodeBackend for centralized naming service, managing node and actor registrations.
• Updated ActorLifecycle and SystemMessageHandler to utilize the new backend structure.
• Enhanced documentation and tests for the new naming backend implementations.

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• closes GitHub issue: #xxx

[codecov-commenter]

Codecov Report

❌ Patch coverage is 67.49380% with 262 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/pulsing-actor/src/cluster/backends/head.rs	69.21%	177 Missing ⚠️
crates/pulsing-actor/src/system/handler.rs	62.50%	27 Missing ⚠️
crates/pulsing-actor/src/system/config.rs	43.58%	22 Missing ⚠️
crates/pulsing-actor/src/transport/http2/server.rs	35.48%	20 Missing ⚠️
...rates/pulsing-actor/src/cluster/backends/gossip.rs	76.66%	14 Missing ⚠️
crates/pulsing-actor/src/transport/http2/client.rs	0.00%	2 Missing ⚠️

Files with missing lines	Coverage Δ
crates/pulsing-actor/src/system/mod.rs	80.95% <100.00%> (+6.90%)	⬆️
crates/pulsing-actor/src/watch.rs	90.04% <100.00%> (ø)
crates/pulsing-actor/src/transport/http2/client.rs	59.43% <0.00%> (-0.12%)	⬇️
...rates/pulsing-actor/src/cluster/backends/gossip.rs	76.66% <76.66%> (ø)
crates/pulsing-actor/src/transport/http2/server.rs	64.10% <35.48%> (-0.46%)	⬇️
crates/pulsing-actor/src/system/config.rs	33.14% <43.58%> (+8.50%)	⬆️
crates/pulsing-actor/src/system/handler.rs	53.88% <62.50%> (+5.26%)	⬆️
crates/pulsing-actor/src/cluster/backends/head.rs	69.21% <69.21%> (ø)

... and 1 file with indirect coverage changes

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