Add option of passing in an object(SpanOptions) to the startChildSpan

[mayurkale22]

This will make things consistent with startRootSpan function. Currently, startChildSpan function takes three arguments instead of an object.

To avoid breaking changes, I have introduced three favors of startChildSpan overloaded function.

[codecov-io]

Codecov Report

Merging #349 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #349      +/-   ##
==========================================
- Coverage   95.28%   95.27%   -0.02%     
==========================================
  Files         124      124              
  Lines        8339     8361      +22     
  Branches      619      619              
==========================================
+ Hits         7946     7966      +20     
- Misses        393      395       +2

Impacted Files	Coverage Δ
src/stackdriver-monitoring.ts	81.05% <0%> (-2.11%)	⬇️
test/test-root-span.ts	100% <0%> (ø)	⬆️
test/test-tracer.ts	100% <0%> (ø)	⬆️
src/trace/model/tracer.ts	84.26% <0%> (ø)	⬆️
test/test-instana.ts	100% <0%> (ø)	⬆️
src/trace/model/root-span.ts	100% <0%> (ø)	⬆️
src/trace/model/types.ts	100% <0%> (ø)	⬆️
test/test-zipkin.ts	100% <0%> (ø)	⬆️
src/mongodb.ts	92.75% <0%> (ø)	⬆️
test/test-zpages.ts	99.62% <0%> (ø)	⬆️

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[mayurkale22]

@draffensperger Thanks for the reviews