Add authentication and custom intercept support.

[jmjoy]

1. Add GrpcReporter::authentication method.
2. Add GrpcReporter::with_custom_intercept method.
3. Adjust the logic to suit the changes.

[codecov-commenter]

Codecov Report

Merging #50 (21b9f2a) into master (4c61a5b) will decrease coverage by 3.86%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
- Coverage   21.40%   17.54%   -3.87%     
==========================================
  Files          20       17       -3     
  Lines         612      570      -42     
==========================================
- Hits          131      100      -31     
+ Misses        481      470      -11     

Impacted Files	Coverage Δ
src/reporter/grpc.rs	0.00% <0.00%> (ø)
src/skywalking_proto/v3/mod.rs	0.00% <ø> (ø)
src/trace/span.rs	0.00% <0.00%> (ø)
tests/propagation.rs
tests/management.rs
tests/trace_context.rs

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[wu-sheng]

Please update docs about these new features.

[jmjoy]

@wu-sheng The authentication feature is described in the example.