feat: [sc-38551] add fieldIsPresent to flatc generated code

[tayloraswift]

Thank you for submitting a PR!

Please delete this standard text once you've created your own description.

If you make changes to any of the code generators (src/idl_gen*) be sure to
build your project, as it will generate code based on the changes. If necessary
the code generation script can be directly run (scripts/generate_code.py),
requires Python3. This allows us to better see the effect of the PR.

If your PR includes C++ code, please adhere to the
Google C++ Style Guide,
and don't forget we try to support older compilers (e.g. VS2010, GCC 4.6.3),
so only some C++11 support is available.

For any C++ changes, please make sure to run sh scripts/clang-format-git.sh

Include other details as appropriate.

Thanks!

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.