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

CMakeLists.txt

@@ -821,6 +821,9 @@ if(CURL_USE_OPENSSL)
   endif()
 
   if(HAVE_BORINGSSL)
+    if(BORINGSSL_VERSION)
+      set(CURL_BORINGSSL_VERSION "\"${BORINGSSL_VERSION}\"")
+    endif()
     set(_openssl "BoringSSL")
   elseif(HAVE_AWSLC)
     set(_openssl "AWS-LC")