cmake: add BORINGSSL_VERSION configuration variable, test in CI

To simplify setting BoringSSL version, using:
`-DBORINGSSL_VERSION=0.20260211.0`
or
`-DBORINGSSL_VERSION=${boringssl_version}`

Previously it could be set via C flags, using complicated shell quotes:
`-DCMAKE_C_FLAGS="-DCURL_BORINGSSL_VERSION=\\\"${boringssl_version}\\\""`
(the C flags method remains, also for autotools)

It'd be nice if BoringSSL published its version not just via
`MODULE.bazel` in its source tree, but from its public headers, to make
these workarounds unnecessary.

Also:
- GHA/http3-linux: test both options.

Closes #20571

lib/curl_config-cmake.h.in

@@ -327,6 +327,9 @@
 /* MIT Kerberos version */
 #cmakedefine CURL_KRB5_VERSION ${CURL_KRB5_VERSION}
 
+/* BoringSSL version */
+#cmakedefine CURL_BORINGSSL_VERSION ${CURL_BORINGSSL_VERSION}
+
 /* Define to 1 if you have the <ifaddrs.h> header file. */
 #cmakedefine HAVE_IFADDRS_H 1