Adding a full, end-to-end example of propagation at work in the
example application, including a test.

Adding the use of propagators into the integrations.

Fixing incorrect type annotation for WSGI getter for
HTTPTextFormatter.

Using setdefautl to enable more atomic and simple
setting of headers.

Using UnitTest style assertions vs PyTest.

Make getter/setter types specific to context

Renaming propagator to plural, to match other top level module names.

Using static functions for setting / getting global httptextformat,
and making propagator methods static.

Renaming "TraceStateHTTP" to "TraceContextHTTP", since
the latter is the correct name for the w3c spec.