Add a new "not sampled" mask to complement the existing "sampled" one.

Rename `FlagsUnused` to `FlagsUnset`.

Add documentation for each of the flags to help understand their
purpose.

The B3 specification states "Debug is encoded as `X-B3-Flags: 1`. Absent
or any other values can be ignored", so testing of other values should
not result in an error.

Remove test cases that would fail if the fallback header format was
expected to not be used.

Add the B3Encoding and valid HTTP based values. Change the B3 propagator
to use these bitmask fields to specify the inject encoding it will
propagate.

Add a FlagsDebug and FlagsDeferred to track the B3 trace state.

Add helper methods to the SpanContext to check the debug and deferred
bit of the trace flags.

Update SpanContext.IsSampled to return if the sampling decision is to
sample rather than if the sample bit is set. This means that if the
debug bit is also set it will return true.

This check makes sample only headers not propagate.

Use the passed SpanContext and check directly the span ID.

Run update checked SpanID to match sent.

Add tests to validate sample only transmissions and debug flag support.

This is no longer the parent.

The B3SingleHeader name will conflict with the upcoming change to prefix
the SingleHeader encoding with "B3". There are a few options to address
this conflict, but in the end we do not need to be exporting these
values. They are duplicates of the OpenZipkin package and users should
use those.

Include a `B3` prefix to scope the encoding names.

Move the related support method to the B3Encoding itself, instead of the
B3 propagator.

Add tests to provide a sanity check for encoding bitmasks.

Update test name to better describe how unused bits have no affect on
the sampling decision. Include the inverse of this test as well: not
sampled but has unused bits.

Co-authored-by: Anthony Mirabella <a9@aneurysm9.com>