oauth: Rename macro to avoid collisions on Windows

oauth: Rename macro to avoid collisions on Windows

Our json parsing defined the macros OPTIONAL and REQUIRED to decorate the
structs with for increased readability. This however collides with macros
in the <windef.h> header on Windows.

../src/interfaces/libpq/fe-auth-oauth-curl.c:398:9: warning: "OPTIONAL" redefined
  398 | #define OPTIONAL false
      |         ^~~~~~~~
In file included from D:/a/_temp/msys64/ucrt64/include/windef.h:9,
                 from D:/a/_temp/msys64/ucrt64/include/windows.h:69,
                 from D:/a/_temp/msys64/ucrt64/include/winsock2.h:23,
                 from ../src/include/port/win32_port.h:60,
                 from ../src/include/port.h:24,
                 from ../src/include/c.h:1331,
                 from ../src/include/postgres_fe.h:28,
                 from ../src/interfaces/libpq/fe-auth-oauth-curl.c:16:
include/minwindef.h:65:9: note: this is the location of the previous definition
   65 | #define OPTIONAL
      |         ^~~~~~~~

Rename to avoid compilation errors in anticipation of implementing
support for Windows.

Reported-by: Dave Cramer (on PostgreSQL Hacking Discord)