feat: introduce AggregatorSelector type

[legendecas]

Which problem is this PR solving?

• Introduce AggregatorSelector term in https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/metrics/sdk.md#sdk-terminology.
• Removes dependencies between metrics and processors.
• Metrics now asks aggregator selector interface for aggregators (not requiring the whole processor interface).

Short description of the changes

• A new AggregatorSelector interface.
• Processors implements AggregatorSelector interface.

[codecov]

Codecov Report

Merging #1712 (d740c23) into master (781b30f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1712   +/-   ##
=======================================
  Coverage   91.38%   91.38%           
=======================================
  Files         164      164           
  Lines        5059     5059           
  Branches     1048     1048           
=======================================
  Hits         4623     4623           
  Misses        436      436           

Impacted Files	Coverage Δ
...ges/opentelemetry-metrics/src/SumObserverMetric.ts	100.00% <ø> (ø)
...entelemetry-metrics/src/UpDownSumObserverMetric.ts	100.00% <ø> (ø)
...s/opentelemetry-metrics/src/ValueObserverMetric.ts	100.00% <ø> (ø)
...es/opentelemetry-metrics/src/BaseObserverMetric.ts	100.00% <100.00%> (ø)
...s/opentelemetry-metrics/src/BatchObserverMetric.ts	93.10% <100.00%> (ø)
...ackages/opentelemetry-metrics/src/CounterMetric.ts	100.00% <100.00%> (ø)
...s/opentelemetry-metrics/src/UpDownCounterMetric.ts	100.00% <100.00%> (ø)
...s/opentelemetry-metrics/src/ValueRecorderMetric.ts	100.00% <100.00%> (ø)
...ages/opentelemetry-metrics/src/export/Processor.ts	92.00% <100.00%> (ø)

[legendecas]

Closed due to inactivity. I will continue the investigation once there are any updates on the spec.