Add management support.

[jmjoy]

1. Add report instance properties support.
2. Add keep alive support.
3. Replace function params ToString to Into<String> for better semantics.
4. Upgrade min rust version requirement.

[codecov-commenter]

Codecov Report

Merging #48 (41d7af1) into master (10aa902) will decrease coverage by 0.72%.
The diff coverage is 7.31%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
- Coverage   22.12%   21.40%   -0.73%     
==========================================
  Files          18       20       +2     
  Lines         592      612      +20     
==========================================
  Hits          131      131              
- Misses        461      481      +20     

Impacted Files	Coverage Δ
src/logging/logger.rs	0.00% <0.00%> (ø)
src/logging/record.rs	0.00% <0.00%> (ø)
src/management/instance.rs	0.00% <0.00%> (ø)
src/metrics/meter.rs	0.00% <0.00%> (ø)
src/metrics/metricer.rs	0.00% <0.00%> (ø)
src/reporter/grpc.rs	0.00% <0.00%> (ø)
src/reporter/mod.rs	0.00% <ø> (ø)
src/reporter/print.rs	36.84% <ø> (ø)
src/skywalking_proto/v3/mod.rs	0.00% <ø> (ø)
src/trace/span.rs	0.00% <ø> (ø)
... and 4 more

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[wu-sheng]

Do the properties of instance support customization? Such as using a JSON to inject some flags to instance properties?