feat: batch span processor environment config

[mwear]

Which problem is this PR solving?

• Fixes Batch Span Processor environment config #1677 (at least partially)

Short description of the changes

• This PR adds environment variable configuration to the batch span processor for two of the four environment variables defined in the spec: OTEL_BSP_SCHEDULE_DELAY_MILLIS and OTEL_BSP_MAX_EXPORT_BATCH_SIZE.
• There are two additional environment variables defined in the spec that do not apply to the current JS BatchSpanProcessor implementation: OTEL_BSP_EXPORT_TIMEOUT_MILLIS and OTEL_BSP_MAX_QUEUE_SIZE. We don't enforce a max queue size, or a timeout on export. We can discuss whether or not we should, and if that work should be part of this PR or not.
• The adopts the spec defaults of 512 spans per batch, and a delay of 5000ms, which is different than the previous values of 100 spans and 20000ms. With these changes we will send more spans, more frequently.

[codecov]

Codecov Report

Merging #1755 (afdb51a) into master (d77a8b9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1755   +/-   ##
=======================================
  Coverage   92.18%   92.18%           
=======================================
  Files         165      165           
  Lines        5594     5595    +1     
  Branches     1204     1206    +2     
=======================================
+ Hits         5157     5158    +1     
  Misses        437      437           

Impacted Files	Coverage Δ
...ckages/opentelemetry-core/src/utils/environment.ts	87.23% <ø> (ø)
...telemetry-tracing/src/export/BatchSpanProcessor.ts	90.90% <100.00%> (+0.13%)	⬆️