Ports: Add flatbuffers library (#6050)

The flatbuffers library is a serialization library, created by Google
for game development and performance-critical applications.
It aims to be fast and efficient.

This commit creates a port of it to SerenityOS.

The flatbuffers build process generates three things: some header files,
a library (libflatbuffers) and a schema compiler (flatc).

There are tests, but they are not compiled, because it runs the
flatbuffers schema compiler, one of the things we are cross-compiling.
The compiler will not run because the target is different from the host

Ports/flatbuffers/package.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env -S bash ../.port_include.sh
+
+port="flatbuffers"
+version="1.12.0"
+auth_type=sha256
+files="https://github.com/google/flatbuffers/archive/refs/tags/v${version}.tar.gz v${version}.tar.gz 62f2223fb9181d1d6338451375628975775f7522185266cd5296571ac152bc45"
+useconfigure=true
+# Since we are cross-compiling, we cannot build the tests, because we need
+# the flatbuffers compiler to build them
+configopts="-DCMAKE_TOOLCHAIN_FILE=$SERENITY_ROOT/Toolchain/CMakeToolchain.txt -DFLATBUFFERS_BUILD_TESTS=off"
+
+configure() {
+    run cmake $configopts
+}
+
+install() {
+    run make install
+}