openssl: add `CURL_BORINGSSL_VERSION` to identify BoringSSL

BoringSSL doesn't keep a version number, and doesn't self-identify itself
via any other revision number via its own headers. We can identify
BoringSSL revisions by their commit hash. This hash is typically known by
the builder. This patch adds a way to pass this hash to libcurl, so that
it can display in the curl version string:

For example:

`CFLAGS=-DCURL_BORINGSSL_VERSION="c239ffd0"`

```
curl 7.84.0 (x86_64-w64-mingw32) libcurl/7.84.0 BoringSSL/c239ffd0 (Schannel) zlib/1.2.12 [...]
Release-Date: 2022-06-27
Protocols: dict file ftp ftps gopher gophers http https imap imaps ldap ldaps mqtt pop3 [...]
Features: alt-svc AsynchDNS brotli gsasl HSTS HTTP2 HTTP3 HTTPS-proxy IDN IPv6 Kerberos [...]
```

The setting is optional, and if not passed, BoringSSL will appear without
a version number, like before this patch.

Closes #9113

lib/vtls/openssl.c

@@ -4454,7 +4454,13 @@ static size_t ossl_version(char *buffer, size_t size)
                    (LIBRESSL_VERSION_NUMBER>>12)&0xff);
 #endif
 #elif defined(OPENSSL_IS_BORINGSSL)
+#ifdef CURL_BORINGSSL_VERSION
+  return msnprintf(buffer, size, "%s/%s",
+                   OSSL_PACKAGE,
+                   CURL_BORINGSSL_VERSION);
+#else
   return msnprintf(buffer, size, OSSL_PACKAGE);
+#endif
 #elif defined(HAVE_OPENSSL_VERSION) && defined(OPENSSL_VERSION_STRING)
   return msnprintf(buffer, size, "%s/%s",
                    OSSL_PACKAGE, OpenSSL_version(OPENSSL_VERSION_STRING));