Add ability to select certificate based on ClientHello random bytes

[mingyech]

Description

This PR lets clients to modify the client hello random bytes and lets servers to choose certificates based on the ClientHello random bytes using the GetCertificate function.

This serves a similar purpose to ServerName in the ClientHello, where the server can use different certificate based on the ServerName in the ClientHello. This change would be useful for censorship resistance settings where the client does not want to add extra extension like ServerName in the ClientHello to prevent censors from fingerprinting the connection.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.09%. Comparing base (eddca22) to head (0a2438a).

Files	Patch %	Lines
state.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #583      +/-   ##
==========================================
+ Coverage   80.07%   80.09%   +0.02%     
==========================================
  Files         101      101              
  Lines        5324     5330       +6     
==========================================
+ Hits         4263     4269       +6     
  Misses        686      686              
  Partials      375      375              

Flag	Coverage Δ
go	80.12% <66.66%> (+0.02%)	⬆️
wasm	63.90% <66.66%> (+<0.01%)	⬆️

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