Support Key generate Func in ServiceEvent

[cvictory]

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

[codecov-commenter]

Codecov Report

Merging #1286 (ba494dd) into 3.0 (968650f) will decrease coverage by 4.77%.
The diff coverage is 55.38%.

@@            Coverage Diff             @@
##              3.0    #1286      +/-   ##
==========================================
- Coverage   59.53%   54.75%   -4.78%     
==========================================
  Files         259      277      +18     
  Lines       12737    13156     +419     
==========================================
- Hits         7583     7204     -379     
- Misses       4199     5040     +841     
+ Partials      955      912      -43     

Impacted Files	Coverage Δ
cluster/cluster_impl/available_cluster_invoker.go	66.66% <ø> (ø)
cluster/cluster_impl/broadcast_cluster_invoker.go	76.47% <ø> (ø)
cluster/cluster_impl/failfast_cluster_invoker.go	66.66% <ø> (ø)
cluster/cluster_impl/mock_cluster.go	0.00% <0.00%> (ø)
cluster/loadbalance/random.go	100.00% <ø> (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)
common/extension/config_center_factory.go	0.00% <ø> (ø)
... and 291 more

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