growpart: Specify partition without partition number

Instead of taking an argument for the device and partition number, `growpart` should have an option to specify the partition without the partition number (e.g via the full partition device name, label or UUID).

We're currently making use of `growpart` to resize the `/var` partition on a fleet of heterogeneous bare metal servers. Some partitions are on hard drives while others are on NVMe drives, so we currently need to carry around a script that determines the device name and partition number for the `/var` partition before running `growpart`.

The complexity stems from there seemingly being no standard command that will give you the partition number for a device. Since this can be messy to implement, it seems as though others might benefit from this being made simpler.

The scenario is roughly this:
- Machine boots up
- The script has no idea what the naming format of the device will be
- The script does know the partition label and the mount point that it needs to resize

Currently we have to do something like this:

```bash
SOURCE=$(findmnt -nr -o SOURCE -T /var)
DEVICE=/dev/$(lsblk -nr -o PKNAME "$SOURCE")
PARTNUM=${SOURCE#$DEVICE}
PARTNUM=${PARTNUM#p}
growpart "$DEVICE" "$PARTNUM"
```

Though it's actually worse in reality because we don't have `lsblk` available, so it's a bunch of brittle string manipulation instead.

The improvement would be to be able to resize using one of the following options:

```bash
SOURCE=$(findmnt -nr -o SOURCE -T /var)
growpart --device "$SOURCE"
```

```bash
UUID=$(findmnt -nr -o UUID -T /var)
growpart --uuid "$UUID"
```

```bash
growpart --label VAR
```

The reason to throw in the label and UUID options is that `blkid` (which is already a dependency of `growpart`) can get the device name of the partition with `blkid --label` and `blkid --uid`. I think this would be enough to implement any of the above options in `growpart`.

I'm open to other approaches to solving this, I'm mostly just after a way to avoid specifying the partition number in a separate argument because it doesn't lend itself to use in scripts where the partition needs to be discovered first. Determining the partition number has a few catches around device naming formats and is just generally a bit error prone.