Add a `summarize` action that creates a single issue for all resources that satisfy the conditions

Intro

We have a few requirements to generate a list of issues, e.g., gitlab-org/quality/team-tasks#35, gitlab-org/quality/team-tasks#16, and gitlab-org/release-tools#196. Presumably customers might also need to do this.

Proposal

The idea is to add a new action that would create a single new issue for all resources instead of updating each resource.

It would have three elements, each of which accept placeholders:

• title: the title of the new issue
• summary: the content of the new issue
• item: a string that represents each single resource

They would allow placeholders like the comment action does, plus:

• resource_url: a link to the resource
• resource_title: the resource's title
• items: the collection of items to be included in the summary. Each will appear on its own line

It could look like:

actions:
  summarize:
    title: Issues to triage
    item: - [ ] [{{resource_title}}]({{resource_url}}) {{labels}}
    summary: |
      Here is a list of issues to triage:
      {{items}}

Which would generate a new issue with the content:

Here is a list of issues to triage:
- [ ] [Issue 1]()
- [ ] [Issue 2]()
- [ ] [Issue 3]()

/cc @meks