[mbedtls] change mbedtls to git submodule (#12107)

This commit changes the mbedtls repo in openthread from source code to
git submodule.

This makes it easier for mbedtls version upgrade. This PR doesn't
upgrade the mbedtls version. v.3.6.0 is stil used to ensure nothing is
broken. The original OT specific build files (BUILD.gn, CMakeLists.txt
and config) are kept and unchanged. I've verified that the headers and
sources in the list of BUILD.gn are correct.

script/bootstrap

@@ -157,6 +157,7 @@ install_packages()
 
 main()
 {
+    git submodule update --init --recursive || true
     install_packages
     echo 'bootstrap completed successfully.'
 }