CDRIVER-6091 append C++ compiler to handshake

[kevinAlbs]

Summary

Append detected C++ compiler to handshake.

Verified with this patch build: https://spruce.mongodb.com/version/68c08e74190e6700073c344b

Background & Motivation

See CDRIVER-6091. Knowing if a C++ compiler was detected may help inform future decisions.

Example from the handshake before:

{
    "driver": {
        "name": "mongoc",
        "version": "2.2.0-pre"
    },
    "os": {
        "type": "Linux",
        "name": "Ubuntu",
        "version": "22.04",
        "architecture": "x86_64"
    },
    "platform": "cfg=0x06235e88e9 posix=200809 stdc=199901 CC=GCC 11.4.0 CFLAGS=\"\" LDFLAGS=\"\""
}

This PR adds CXX to the platform string of the MongoDB Handshake:

                                                        vvvvvvvvvvvvvv
cfg=0x06235e88e9 posix=200809 stdc=199901 CC=GCC 11.4.0 CXX=GNU 11.4.0 CFLAGS=\"\" LDFLAGS=\"\"
                                                        ^^^^^^^^^^^^^^

If a C++ compiler is not found, CXX=Unknown is included. Alternatively: CXX could be omitted if not detected. But I expect it may help with analysis to easily distinguish between: "driver too old to check" and "checked and did not find".

[connorsmacd]

LGTM

[eramongodb]

Suggest adding a comment to the check_language (CXX) -> enable_language (CXX) in the root CMakeLists.txt documenting that these optional compiler ID + version handshake metadata is dependent on them (no longer just for "some C++-specific tests"):

mongo-c-driver/CMakeLists.txt

Lines 229 to 240 in 340b1f4

	# Optionally enable C++ to do some C++-specific tests
	include (CheckLanguage)
	check_language (CXX)
	if (CMAKE_CXX_COMPILER)
	enable_language (CXX)
	if (NOT CMAKE_CXX_STANDARD)
	# Default to C++11 for purposes of testing.
	set (CMAKE_CXX_STANDARD 11)
	endif ()
	else ()
	message (STATUS "No CXX support")
	endif ()

[kevinAlbs]

Good idea. Comment updated.