feat: add InMemorySpanExporter, MultiSpanProcessor

[mayurkale22]

Which problem is this PR solving?

• This PR is a follow up of feat: add SimpleSpanProcessor #218 and based on the specs Add SDK span processors spec opentelemetry-specification#205

Short description of the changes

• This PR introduces the InMemorySpanExporter, it can be used for testing purposes, especially in the plugins. It stores the exported spans in a list in memory.
• SDK Usage (example):

  const memoryExporter = new InMemorySpanExporter();
  const tracer = new BasicTracer({
    scopeManager: new NoopScopeManager(),
  });
  // register simple processor with InMemoryExporter
   tracer.addSpanProcessor(new SimpleSpanProcessor(memoryExporter));

  // register batch processor with Jaeger Exporter
  // tracer.addSpanProcessor(new BatchSpanProcessor(jaegerExporter));

• I will open another PR to add Batching processor (BatchSpanProcessor).

[codecov-io]

Codecov Report

Merging #234 into master will decrease coverage by <.01%.
The diff coverage is 99.2%.

@@            Coverage Diff             @@
##           master     #234      +/-   ##
==========================================
- Coverage   98.78%   98.78%   -0.01%     
==========================================
  Files          55       60       +5     
  Lines        2146     2221      +75     
  Branches      147      152       +5     
==========================================
+ Hits         2120     2194      +74     
- Misses         26       27       +1

Impacted Files	Coverage Δ
...entelemetry-basic-tracer/src/MultiSpanProcessor.ts	100% <100%> (ø)
...ages/opentelemetry-basic-tracer/src/BasicTracer.ts	98.36% <100%> (+0.21%)	⬆️
...ry-basic-tracer/src/export/InMemorySpanExporter.ts	100% <100%> (ø)
...metry-basic-tracer/test/MultiSpanProcessor.test.ts	100% <100%> (ø)
packages/opentelemetry-basic-tracer/src/Span.ts	100% <100%> (ø)	⬆️
...ic-tracer/test/export/InMemorySpanExporter.test.ts	100% <100%> (ø)
...pentelemetry-basic-tracer/src/NoopSpanProcessor.ts	75% <75%> (ø)
...kages/opentelemetry-basic-tracer/test/Span.test.ts	100% <0%> (ø)	⬆️
...sic-tracer/test/export/SimpleSpanProcessor.test.ts	100% <0%> (ø)	⬆️
... and 4 more

[bg451]

stopped = false?

[mayurkale22]

reset is just to clean the collected spans. If we make it stopped = false, it won't export/collect any spans after reset.

[OlivierAlbertini]

LGTM. Good job!

[mayurkale22]

/cc @hekike Please review. You may need to make a few minor changes in Zipkin exporter PR (#192) based on this.