Enum support in SettingsGroup derive macro, implemented key attribute

[mpowell90]

Objective

Implements part of #23302

For background on bevy settings see the initial PR:
#23034

Solution

Supports unit-like Enum resources:

#[derive(Resource, SettingsGroup, Reflect, Debug, Default)]
#[reflect(Resource, SettingsGroup, Default)]
enum CounterRefreshRateSettings {
    #[default]
    Slow,
    Fast,
}

Assuming the above is initialised as CounterRefreshRateSettings::Slow, results in:

[counter_refresh_rate_settings]
counter_refresh_rate_settings = "Slow"

Setting the group attribute works the same as structs:

#[settings_group(group = "counter_settings")]

Results in:

[counter_settings]
counter_refresh_rate_settings = "Slow"

This PR adds a key attribute which can only be used on enums (for now), otherwise a compile-time error is thrown:

#[settings_group(key = "refresh_rate")]

Results in:

[counter_refresh_rate_settings]
refresh_rate = "Slow"

Testing

• Added tests that checks merging enums into groups and round trip serialisation.
• Added a "cheat sheet" for the derive syntax, immitating other derive macros in the file.

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-23719

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.

[viridia]

Awesome work!

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-23719

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.