support http2 for java client

[Nicole00]

close #538

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 50.56180% with 44 lines in your changes missing coverage. Please review.

Project coverage is 63.02%. Comparing base (3429ca1) to head (34fdd4f).
Report is 25 commits behind head on master.

Files with missing lines	Patch %	Lines
...vesoft/nebula/client/graph/net/SyncConnection.java	44.44%	23 Missing and 2 partials ⚠️
.../vesoft/nebula/client/graph/SessionPoolConfig.java	41.66%	7 Missing ⚠️
...va/com/vesoft/nebula/client/graph/SessionPool.java	33.33%	3 Missing and 1 partial ⚠️
...ebula/client/graph/net/RoundRobinLoadBalancer.java	63.63%	4 Missing ⚠️
...m/vesoft/nebula/client/graph/NebulaPoolConfig.java	50.00%	2 Missing ⚠️
.../src/main/java/com/vesoft/nebula/util/SslUtil.java	60.00%	2 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #543      +/-   ##
============================================
- Coverage     63.42%   63.02%   -0.40%     
- Complexity      795      801       +6     
============================================
  Files            72       72              
  Lines          3877     3946      +69     
  Branches        535      539       +4     
============================================
+ Hits           2459     2487      +28     
- Misses         1051     1089      +38     
- Partials        367      370       +3     

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