Configure the `maintenanceMode` flag type to be `boolean | Variant` and
update the env parsing to allow passing strings from the env.
The [first
impl](3bbfc9e681)
required you to set a full, variant -- stringified as json -- in the
env, but this is both error-prone and not very user friendly.
Additionally, the name of the variant isn't really important, and if
you're passing a string, you probably want it to be true.
As such, the [second
impl](c38357baa4)
updates the env parsing to read the full string value into a
pre-formatted variant if it's not parseable as a boolean.
As such, to set a custom message, you can now do:
```sh
UNLEASH_EXPERIMENTAL_MAINTENANCE_MODE='Custom message from plain env var string' yarn dev
```
With the [updates to the
UI](https://github.com/Unleash/unleash/pull/10961), it'll look a little
something like this:
<img width="388" height="64" alt="image"
src="https://github.com/user-attachments/assets/6b8a174b-d75f-4748-8f1a-1ad4ebce2073"
/>
## Rationale
This allows locking down Unleash instances with a custom message.
Previously, you'd have to use both maintenance mode and a custom banner
for this, but that requires more work to set properly and it shows two
banners, when you really only want the one.
b33abf036b