Add gateway support for HTTPS

[zhaoyadong00]

What this PR does:
The gateway supports external access to HTTPS.
Which issue(s) this PR fixes:
HTTPS is more secure. It is recommended to use HTTPS.

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Merging #213 (72f8a70) into develop (ac2c232) will increase coverage by 0.05%.
The diff coverage is 4.16%.

@@             Coverage Diff             @@
##           develop     #213      +/-   ##
===========================================
+ Coverage    40.50%   40.56%   +0.05%     
===========================================
  Files           39       39              
  Lines         2264     2283      +19     
===========================================
+ Hits           917      926       +9     
- Misses        1255     1267      +12     
+ Partials        92       90       -2     

Impacted Files	Coverage Δ
pkg/pixiu/listener.go	17.32% <0.00%> (-2.87%)	⬇️
pkg/pixiu/pixiu_start.go	0.00% <0.00%> (ø)
pkg/config/config_load.go	70.37% <100.00%> (+7.75%)	⬆️

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[zhaoyadong00]

@cityiron 帮review一下，看还有哪里需要修改的。