add cat-client as optional dependency

[lorgine-li]

通过在apollo-core 模块中，通过provided的方式引入cat-client，这样子可以在core模块中去掉cat相关的反射调用。同时还可以解决由于反射导致的cat 客户端上报心跳多次的问题。

[lorgine-li]

I have read the CLA Document and I hereby sign the CLA

[codecov-commenter]

Codecov Report

Merging #4414 (877fa73) into master (1c4b38e) will increase coverage by 0.14%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #4414      +/-   ##
============================================
+ Coverage     53.22%   53.37%   +0.14%     
  Complexity     2683     2683              
============================================
  Files           490      490              
  Lines         15300    15252      -48     
  Branches       1586     1586              
============================================
- Hits           8144     8140       -4     
+ Misses         6603     6558      -45     
- Partials        553      554       +1     

Impacted Files	Coverage Δ
...pollo/tracer/internals/cat/CatMessageProducer.java	10.00% <0.00%> (+10.00%)	⬆️
...rk/apollo/tracer/internals/cat/CatTransaction.java	0.00% <0.00%> (ø)
...ework/apollo/internals/RemoteConfigRepository.java	85.27% <0.00%> (-3.07%)	⬇️
...racer/internals/DefaultMessageProducerManager.java	66.66% <0.00%> (ø)

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[nobodyiam]

LGTM