feat(serverless): Fix gateway inferred span design

Avoid duplicate expensive context extraction
Avoid subclassing tracing span for serverless but used serverless context element instead to store / track inferred span while keep tracing feature untouched
Improved propagator to not create / capture inferred span context element on invalid data
Rework context element to hold the inferred spans and its captured data
Release captured data as soon as they span start (never read after this point so reclaiming memory)
Refactor context element and propagator into the right package, not context component (product / feature agnostic)
Refactor unit tests

Author: PerfectSlayer