support http2 for java client

[Nicole00]

close #538

[codecov-commenter]

Codecov Report

Patch coverage: 50.56% and project coverage change: -0.66% ⚠️

Comparison is base (3429ca1) 63.42% compared to head (a40bcde) 62.77%.

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Additional details and impacted files

@@             Coverage Diff              @@
##             master     #543      +/-   ##
============================================
- Coverage     63.42%   62.77%   -0.66%     
- Complexity      795      797       +2     
============================================
  Files            72       72              
  Lines          3877     3946      +69     
  Branches        535      539       +4     
============================================
+ Hits           2459     2477      +18     
- Misses         1051     1097      +46     
- Partials        367      372       +5     

Files Changed	Coverage Δ
...com/vesoft/nebula/client/graph/net/Connection.java	100.00% <ø> (ø)
...va/com/vesoft/nebula/client/graph/SessionPool.java	52.51% <33.33%> (-0.76%)	⬇️
.../vesoft/nebula/client/graph/SessionPoolConfig.java	49.43% <41.66%> (-1.22%)	⬇️
...vesoft/nebula/client/graph/net/SyncConnection.java	50.00% <44.44%> (-4.10%)	⬇️
...m/vesoft/nebula/client/graph/NebulaPoolConfig.java	95.12% <50.00%> (-4.88%)	⬇️
.../src/main/java/com/vesoft/nebula/util/SslUtil.java	81.63% <60.00%> (-0.29%)	⬇️
...ebula/client/graph/net/RoundRobinLoadBalancer.java	84.61% <63.63%> (-5.04%)	⬇️
...vesoft/nebula/client/graph/net/ConnObjectPool.java	57.14% <100.00%> (+1.04%)	⬆️
...com/vesoft/nebula/client/graph/net/NebulaPool.java	86.40% <100.00%> (ø)

... and 1 file with indirect coverage changes

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