CMake: add HTTP/3 support (ngtcp2+nghttp3, quiche)

Add three new CMake Find modules (using the curl license, but I grant
others the right to apply the CMake BSD license instead).

This CMake config is simpler than the autotools one because it assumes
ngtcp2 and nghttp3 to be used together. Another difference is that this
CMake config checks whether QUIC is actually supported by the TLS
library (patched OpenSSL or boringssl) since this can be a common
configuration mistake that could result in build errors later.

Unlike autotools, CMake does not warn you that the features are
experimental. The user is supposed to already know that and read the
documentation. It requires a very special build environment anyway.

Tested with ngtcp2+OpenSSL+nghttp3 and quiche+boringssl, both built from
current git master. Use `LD_DEBUG=files src/curl |& grep need` to figure
out which features (libldap-2.4, libssh2) to disable due to conflicts
with boringssl.

Closes #5359

lib/curl_config.h.cmake

@@ -990,6 +990,18 @@ ${SIZEOF_TIME_T_CODE}
 /* to enable NGHTTP2  */
 #cmakedefine USE_NGHTTP2 1
 
+/* to enable NGTCP2 */
+#cmakedefine USE_NGTCP2 1
+
+/* to enable NGHTTP3  */
+#cmakedefine USE_NGHTTP3 1
+
+/* to enable quiche */
+#cmakedefine USE_QUICHE 1
+
+/* Define to 1 if you have the quiche_conn_set_qlog_fd function. */
+#cmakedefine HAVE_QUICHE_CONN_SET_QLOG_FD 1
+
 /* if Unix domain sockets are enabled  */
 #cmakedefine USE_UNIX_SOCKETS