chore: add a task summarize

[suzuki-shunsuke]

This pull request add a cmdx task summarize, outputting a summary of a give pull request.
For now, the feature of this task is very simple but enough.
This task outputs a list of changed packages.

e.g.

$ cmdx sum 29826
+ bash scripts/summarize.sh "$PR_NUMBER"

[gardener/docforge](https://github.com/gardener/docforge) - Scalable build tool for distributed documentation sources

We assume that we run this command at our laptops and paste the result to the pull request description.

Why is this task necessary?

We provide a task new, creating a pull request to add packages.

cmdx new

This task is useful, but it has a problem that it doesn't work well if users don't have the permission to push a commit to origin.

• cmdx new fails if users don't fork aqua-registry #29797

We tried to resolve this issue, but it's a bit hard because this depends on user's environment.

So we changed the mind.
Users would be able to create a pull request easily without cmdx new.
The benefit of cmdx new is to give a good description.
The description is for maintainers mainly.
We decided to abandon cmdx new and complement the benefit by adding a new task creating a good description.
The maintainer can run the task at their laptops.
Maintainers can resolve issues regarding their environment themselves.

TODO

• Update the contribution guide