Allow setting sizeLimit for `plugins` emptyDir

The `InjectPluginVolumePodSpec` function (`internal/cnpgi/operator/lifecycle.go`) creates `plugins` emptyDir volume, but does not set any `sizeLimit` for it, nor does it allow configuring one.

The function checks if the `plugins` volume already exists before creating it, so I checked if the `Cluster` resource allows setting extra volumes, to set a `sizeLimit` that way, but it doesn't.

It would be nice if there was some option to define `sizeLimit` for the `plugins` volume.

For reference, the cloudnative-pg operator has [EphemeralVolumesSizeLimitConfiguration](https://cloudnative-pg.io/docs/1.30/cloudnative-pg.v1#ephemeralvolumessizelimitconfiguration) to configure this for the `shm` and `scratch-data` volumes.