Feature Request: add common snippet to task.yaml that is run before the prepare, execute, and restore sections

Oftentimes we have code like this in our task.yamls:

```yaml
prepare: |
#shellcheck source=tests/lib/nested.sh
. "$TESTSLIB/nested.sh"
prepare_something_from_nested.sh

restore: |
#shellcheck source=tests/lib/nested.sh
. "$TESTSLIB/nested.sh"
restore_something_from_nested.sh

execute: |
#shellcheck source=tests/lib/nested.sh
. "$TESTSLIB/nested.sh"
execute_something_from_nested.sh
```

it would be nice if we could somehow simplify this with just

```yaml
common: |
#shellcheck source=tests/lib/nested.sh
. "$TESTSLIB/nested.sh"

prepare: |
prepare_something_from_nested.sh

restore: |
restore_something_from_nested.sh

execute: |
execute_something_from_nested.sh

```

this would reduce a lot of duplication we have in our spread task.yaml's in snapd.

Start by reading the task.yaml handling and execution entry points in the spread repository, then trace how prepare, execute, and restore sections are run. Check the existing tests for task.yaml parsing and execution; done means a common section is accepted and run before each of those sections, with coverage confirming the behavior.

Written by the indexing model from the issue text.