Store additional fields in context instead of baggage.

[vastin]

Description:

This PR removes baggage propagation from the X-Ray propagator and use new context key to propagate additional fields of X-Ray trace header.

Existing Issue(s):

It is the alternative solution of https://github.com/open-telemetry/opentelemetry-java-contrib/pull/1178. It fixes incompatible issue with the W3C baggage limit of 8192 bytes by removing the baggage propagation and doesn't break X-Ray trace context propagation.

Testing:

Unit test