support http2 and custom headers

[HarrisChu]

1. support http2 by using httpx
2. support custom http header

close #320

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.00000% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 77.23%. Comparing base (dfe1534) to head (6a0077e).

Files	Patch %	Lines
nebula3/gclient/net/Connection.py	60.00%	16 Missing ⚠️
nebula3/gclient/net/SessionPool.py	66.66%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #322      +/-   ##
==========================================
- Coverage   77.52%   77.23%   -0.30%     
==========================================
  Files          18       18              
  Lines        2496     2530      +34     
==========================================
+ Hits         1935     1954      +19     
- Misses        561      576      +15     

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