feat: enable migrate to be dry run

[ale8k]

Enables juju migrate to be dry run. This runs the migrate command up until the final InitiateMigration call, ensuring pre-checks pass.

Checklist

• Code style: imports ordered, good names, simple structure, etc
• Comments saying why design decisions were made
• Go unit tests, with comments saying what you're testing
• Integration tests, with comments saying what you're testing
• doc.go added or updated in changed packages

QA steps

juju bootstrap lxd dryrun
juju add-model test
juju deploy ubuntu --constraints="arch=arm64"
juju bootstrap lxd dryrun-2
juju switch dryrun
juju migrate test dryrun-2 --dry-run=true

I haven't QA'd a precheck failure but I think it'll be ok as it is covered by the tests.

Links

Launchpad bug: https://bugs.launchpad.net/juju/+bug/

Jira card: JUJU-

[kian99]

Looks good, thanks!

[luci1900]

Looks fine.

Possibly more testing that necessary.