release: create release campaigns

Created by: bobheadxi

Adds basic campaigns integration into our release process (also requires review) by using the importChangesets feature on pull requests that our release tooling already generates.

• release:publish now generates a new campaign
  • we do not use src-cli to generate the changes right now, since it is not currently possible without per-repository steps - this might be enabled by @mrnugget 's https://github.com/sourcegraph/src-cli/pull/361 , this would be really cool
• release:add-to-campaign adds an existing change to release-tracking campaign, i.e. to track a blog post PR like https://github.com/sourcegraph/about/pull/1797:

  yarn run release release:add-to-campaign 3.21.0 sourcegraph/about 1797

Closes https://github.com/sourcegraph/sourcegraph/issues/14971

---

To demo what this might look like, I created an example release-sourcegraph-3.21.0 using "fake" created changesets:

// yarn run release _test:campaign-create-from-changes "$(cat ./.secrets/import.json)"
{
  "name": "release-sourcegraph-3.21.0",
  "description": "[TEST] Track publishing of [email protected]",
  "changes": [
    {
      "repository": "sourcegraph/sourcegraph",
      "pullRequestNumber": 14880
    },
    {
      "repository": "sourcegraph/deploy-sourcegraph",
      "pullRequestNumber": 1108
    },
    {
      "repository": "sourcegraph/deploy-sourcegraph-dot-com",
      "pullRequestNumber": 3705
    }
  ]
}

and added a few other PRs to it using release:add-to-campaign:

Opened https://github.com/sourcegraph/sourcegraph/issues/15039 , https://github.com/sourcegraph/sourcegraph/issues/15045 in response to some things I ran into while working on this