Apply __cdecl to all exported callbacks

[kevinAlbs]

Summary

Apply missing BSON_CALL to all exported callbacks to ensure __cdecl is applied when building with MSVC.

Resolves CDRIVER-2737 and CDRIVER-5678.

Verified with this patch build.

Background & Motivation

Existing expectations

This PR assumes libbson and libmongoc do not support building with a non-cdecl calling convention. Attempting to build libbson with /Gv to specify the __vectorcall calling convention results in errors:

src\libbson\src\bson\bson-clock.c(45): error C2373: 'bson_gettimeofday': redefinition; different type modifier
(... and many more ...)

This PR intends to ensure consumers building with MSVC see __cdecl in the function declaration when building headers to match the expected calling convention of libmongoc and libbson.

Skipping CI testing

Similar to mongodb/mongo-cxx-driver#1202, an attempt was made to build test-libmongoc with /Gv to test a non-cdecl consumer.

Doing so expectedly appears to require marking all callback functions defined in tests with __cdecl to avoid errors like:

src\libmongoc\tests\test-mongoc-command-monitoring.c(131): error C2440: 'function': cannot convert from 'void (__vectorcall *)(const mongoc_apm_command_failed_t *)' to 'mongoc_apm_command_failed_cb_t'
(... and many more ...)

And also appears to require marking functions in private headers used by tests to be marked __cdecl:

test-libmongoc-lib.lib(runner.obj) : error LNK2001: unresolved external symbol _mongoc_array_append_vals@@24
(... and many more ...)

To expedite the fix, tests in CI are not added.

[eramongodb]

The pointer-to-function data members in mongoc_stream_t also require __cdecl.

[kevinAlbs]

The pointer-to-function data members in mongoc_stream_t also require __cdecl.

Great catch. Updated.