add public functions for AWS trace header

[garasubo]

Fix #885

Add public functions to reuse trace header serialize/deserialize logic in this crate

[garasubo]

Is this failure caused by my change? https://github.com/open-telemetry/opentelemetry-rust/actions/runs/3141646959/jobs/5104324937

I believe this change doesn't affect GRPC related part.

---- build_grpc stdout ----
thread 'main' panicked at 'generated file has changed, please commit the change file and rerun the test', opentelemetry-proto/tests/grpc_build.rs:38:5
error: test failed, to rerun pass '-p opentelemetry-proto --test grpc_build'
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[djc]

No, that failure is unrelated.

[TommyCpp]

I think if you merge the upstream changes the test should be fixed