Support spring bean as mock value

[chickenlj]

<dubbo:reference id="hystrixTestServiceC"
interface="corg.apache.dubbo.demo.HystrixTestServiceC"
mock="hystrixTestServiceCCallback" />

[codecov-commenter]

Codecov Report

Merging #6188 into master will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##             master    #6188      +/-   ##
============================================
- Coverage     60.84%   60.83%   -0.02%     
+ Complexity      492      491       -1     
============================================
  Files           999      999              
  Lines         39898    39909      +11     
  Branches       5744     5747       +3     
============================================
+ Hits          24277    24279       +2     
- Misses        12927    12930       +3     
- Partials       2694     2700       +6     

Impacted Files	Coverage Δ	Complexity Δ
...java/org/apache/dubbo/rpc/support/MockInvoker.java	85.12% <50.00%> (-4.88%)	0.00 <0.00> (ø)
...rg/apache/dubbo/remoting/utils/PayloadDropper.java	15.38% <0.00%> (-30.77%)	0.00% <0.00%> (ø%)
...ng/exchange/support/header/HeartbeatTimerTask.java	73.68% <0.00%> (-5.27%)	0.00% <0.00%> (ø%)
...mmon/threadpool/support/AbortPolicyWithReport.java	85.00% <0.00%> (-5.00%)	0.00% <0.00%> (ø%)
...e/dubbo/remoting/transport/netty/NettyChannel.java	55.68% <0.00%> (-4.55%)	20.00% <0.00%> (-1.00%)
...c/main/java/org/apache/dubbo/rpc/RpcException.java	80.00% <0.00%> (-3.34%)	0.00% <0.00%> (ø%)
...ache/dubbo/remoting/transport/mina/MinaClient.java	56.92% <0.00%> (-1.54%)	8.00% <0.00%> (-1.00%)
...pache/dubbo/registry/support/AbstractRegistry.java	78.06% <0.00%> (-1.49%)	0.00% <0.00%> (ø%)
.../dubbo/remoting/transport/netty4/NettyChannel.java	65.34% <0.00%> (-1.00%)	0.00% <0.00%> (ø%)
...dubbo/remoting/exchange/support/DefaultFuture.java	77.77% <0.00%> (-0.86%)	0.00% <0.00%> (ø%)
... and 6 more

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