add 'Authentication' for grpc request

[limfriend]

Requirement or improvement
Please describe about your requirements or improvement suggestions.
In some service provider user case, they asked for authentication of agent about data uplink. Add an optional config item in agent side, named auth.token. This token will go with every request.

Related issues
apache/skywalking#934

[kezhenxu94]

Thanks, this is what I need. Can you please fix code format and update the doc?

[codecov-commenter]

Codecov Report

Merging #60 into master will increase coverage by 0.13%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   69.91%   70.04%   +0.13%     
==========================================
  Files          14       14              
  Lines         668      671       +3     
==========================================
+ Hits          467      470       +3     
  Misses        160      160              
  Partials       41       41              

Impacted Files	Coverage Δ
reporter/grpc.go	61.74% <66.66%> (+0.63%)	⬆️

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[kezhenxu94]

LGTM