dependencies.

- Renamed security_context -> security_connector.
- Credentials are now responsible for creating their own connectors.

Also removed the unused shared projects.

-) using dupenv_s instead of getenv_s and calling strdup ourselves.
-) few impossible-to-obtain if checks.
-) various signed/unsigned casting.
-) using time_t instead of time32_t
-) checking output of FormatMessage for failures.
-) don't redefine _WIN32_WINNT without undefining it first.
-) fixed msvc's interlocked casting.
-) renamed AddPort to AddListeningPort.
-) added protobuf's third_party includes to search path.
-) added a missing definition for inet_ntop in mingw32.
-) removed useless declarations.

First: ugh.

Second: allow tracer global variables to be registered and then parsed out of a configuration environment variable.

Expose TSI trace config variable directly to ease this a little.

- Renaming default credentials -> google default credentials.
- Various other things in cpp:
  - Adding Cpp wrapping for JWT Tokens.
  - Renaming ComposeCredentials -> CompositeCredentials.

And reflects the C++ API in ServerBuilder.

Remove 'secure_serer_create', and instead attach credentials to ports, meaning different ports can serve different credentials.