Opentelemetry-dotnet: `Baggage` header should be lowercase

[Rast1234]

Bug Report

Currently BaggagePropagator works with header called Baggage:

        internal const string BaggageHeaderName = "Baggage";

Specification explicitly states that header name is baggage (all lowercase).

Currently propagator works for HTTP since headers are case-insensitive, but it can not be used as-is for, say, RabbitMQ. We have to do .ToLower() when getting or setting the value.

[kipwoker]

There was also a similar conversation: https://github.com/dotnet/aspnetcore/pull/28328

[cijothomas]

Fixing this might be considered breaking behavior, but this can be treated as a bug in original release, and hence can be fixed.

[cijothomas]

Will do 1.1.0 release addressing this. (and not 1.0.1 hotfix) - As discussed in 4/20/2021 SIG Meeting.