Add MapCarrier

[MrAlias]

Adds a new MapCarrier type to the propagation package that holds propagated cross-cutting concerns in a map held in memory. This type is motivated by users common need to inject/extract propagated values using a simple, built-in, Go type. For instance, users can create a new MapCarrier with values from the environment and inject the trace context into a Go context.

Supersedes and closes #2245.

Fixes #2243.

[codecov]

Codecov Report

Merging #2334 (709f532) into main (4ba964b) will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #2334     +/-   ##
=======================================
- Coverage   73.6%   73.6%   -0.1%     
=======================================
  Files        175     175             
  Lines      12409   12419     +10     
=======================================
  Hits        9144    9144             
- Misses      3034    3041      +7     
- Partials     231     234      +3     

Impacted Files	Coverage Δ
propagation/propagation.go	85.7% <100.0%> (+4.4%)	⬆️
sdk/metric/refcount_mapped.go	80.0% <0.0%> (-20.0%)	⬇️
...s/otlp/otlptrace/internal/connection/connection.go	14.6% <0.0%> (-1.6%)	⬇️
sdk/metric/sdk.go	80.2% <0.0%> (-1.5%)	⬇️