Add baggage span processor

[martin308]

Adds a baggage span processor as per the other contrib repositories.

This processor takes items from the baggage and adds the items as attributes on spans.

Eg. open-telemetry/opentelemetry-go-contrib#5404

[martin308]

The macOS tests look like they might be flaky but the linux test result is for my new tests will poke around and try to find the cause

[nachoBonafonte]

Could you change it to a more expressible naming, it looks very generic like this
It also would need proper documentation in the source code for the future users, for the rest it LGTM

[martin308]

From the sync, naming wise we will go BaggagePropagationProcessor

[martin308]

Updated with the following changes:

• new name for the processor
• moved to an individual library/target for the processor

I left the contrib tests as a single test target but we could break those up into individual test targets too if we think it is worth it?

[martin308]

I think the only remaining question now is wether we want to have all the contrib tests in one test target or also have many test targets for each piece of contrib

[nachoBonafonte]

Looks good to e, I don't mind having the contribution tests under the same test target or separated