autoinstall layout for guided LUKS encryption

[Ryan Mounce]

Essentially we're looking for the guided LVM/LUKS option available via the subiquity GUI to be extended to autoinstall.

Previously we could achieve this with d-i/preseed:
```
d-i partman-auto/method string crypto
d-i partman-crypto/passphrase password TEMPORARY
d-i partman-crypto/passphrase-again password TEMPORARY
```

So it would be nice to have something like:
```
  storage:
    layout:
      name: lvm
      options:
        encrypt: True
        luks_options:
          password: TEMPORARY
```

[Michael Hudson-Doyle (mwhudson)]

Yes, this makes sense and should be reasonably easy.

[Ryan Mounce (rmounce)]

With 22.04 this is now a blocker for us, legacy netboot images are no longer published so we've lost our fallback for unattended LUKS installation.

[Ryan Mounce (rmounce)]

I've implemented this & posted a PR: https://github.com/canonical/subiquity/pull/1579

```
  storage:
    layout:
      name: lvm_use_all_space
      password: TEMPORARY
```

is equivalent to

```
d-i partman-auto/method string crypto
d-i partman-crypto/passphrase password TEMPORARY
d-i partman-crypto/passphrase-again password TEMPORARY
```