support Key Func in ServiceEvent

[cvictory]

Add Key Func in ServiceEvent, so that the developer can extend the Key generate method.

[codecov-commenter]

Codecov Report

Merging #1285 (7e6dbcb) into 1.5 (a10ff4d) will decrease coverage by 9.11%.
The diff coverage is 59.95%.

@@            Coverage Diff             @@
##              1.5    #1285      +/-   ##
==========================================
- Coverage   66.13%   57.02%   -9.12%     
==========================================
  Files         174      280     +106     
  Lines        9343    13672    +4329     
==========================================
+ Hits         6179     7796    +1617     
- Misses       2541     4913    +2372     
- Partials      623      963     +340     

Impacted Files	Coverage Δ
cluster/cluster_impl/available_cluster_invoker.go	66.66% <ø> (-6.67%)	⬇️
cluster/cluster_impl/broadcast_cluster_invoker.go	76.47% <ø> (-3.53%)	⬇️
cluster/cluster_impl/failfast_cluster_invoker.go	66.66% <ø> (-4.77%)	⬇️
cluster/cluster_impl/mock_cluster.go	0.00% <0.00%> (ø)
cluster/loadbalance/random.go	100.00% <ø> (ø)
cluster/router/healthcheck/factory.go	66.66% <0.00%> (ø)
common/extension/auth.go	0.00% <ø> (ø)
common/extension/cluster.go	0.00% <ø> (ø)
common/extension/cluster_interceptor.go	0.00% <0.00%> (ø)
common/extension/config_center.go	0.00% <ø> (ø)
... and 394 more

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