Create a helper to define overridable configs (#10731)

### Summary
Context: https://github.com/pytorch/executorch/discussions/10661. As
part https://github.com/pytorch/executorch/issues/10716, this creates a
helper to define ET configs.

#### Why define_overridable_config?

This is a simple wrapper to configure an option via the user or fallback
to a default value. Instead of having people do this manually, this is
convenient and consistent. More specifically:

- If the option is set via the CLI or `set`, then save that value in the
cache — with our own description
- If the option is unset, then fallback to the default value

The user options will be loaded *before* loading the `default.cmake`
file. This means, when we load the default file, we can use the user set
options or fallback on defaults. Then finally we can do a final sweet of
option validations. After this point, all options will be frozen and
immutable.

Moreover, having a helper function will allow us to enforce standards on
options. For example, naming standards done in this PR.

#### Migration plan

For now, I've only migrated a simple option `EXECUTORCH_ENABLE_LOGGING`.
We'll incrementally migrate remaining options in future PRs.

### Test plan

For changes to `EXECUTORCH_ENABLE_LOGGING`, I changed the release modes
and checked the finaly `CMakeCache.txt` file. Rely on CI for full tests.

For python tests:

```
$ python3 -m unittest discover -s tools/cmake -p "*.py"
```

I will include this test in CI in the next diff.