Add autoprop package to provide configuration of propagators with useful defaults and envar support

[MrAlias]

Use case

otel.SetTextMapPropagator(autoprop.NewTextMapPropagator())

to replace

otel.SetTextMapPropagator(propagation.NewCompositeTextMapPropagator(propagation.TraceContext{}, propagation.Baggage{}))

• Defaults to TraceContext with Baggage
• Arguments to NewTextMapPropagator act the same as NewCompositeTextMapPropagator and override defaults
• OTEL_PROPAGATORS overrides defaults and arguments
• Custom 3rd-party propagators can be registered with RegisterTextMapPropagator and then supported in the parsed propagator from OTEL_PROPAGATORS

Resolve open-telemetry/opentelemetry-go#1265
Resolve open-telemetry/opentelemetry-go#2304
Related to open-telemetry/opentelemetry-go#1404

[codecov]

Codecov Report

Merging #2258 (ded3cb7) into main (b7910af) will increase coverage by 0.3%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #2258     +/-   ##
=======================================
+ Coverage   73.7%   74.1%   +0.3%     
=======================================
  Files        142     144      +2     
  Lines       6457    6549     +92     
=======================================
+ Hits        4764    4856     +92     
  Misses      1550    1550             
  Partials     143     143             

Impacted Files	Coverage Δ
propagators/autoprop/propagator.go	100.0% <100.0%> (ø)
propagators/autoprop/registry.go	100.0% <100.0%> (ø)

[pellared]

I had a call with @MrAlias and we discussed 👆

The current approach with autoprop is safer as

1. We create less packages - less confusion for the users
2. Having it in contrib allows to make breaking changes (as it is still v0)