Add Simple Changelog Management

[jonathan343]

Summary

Changelog entries are currently being managed manually for each smithy package. This PR includes the following changes to help automate this process and improve consistency:

• Rename changelog files from CHANGES.md to CHANGELOG.md
• Adds a new-entry.py script that creates a new change entry under a package's .changes/next-release directory.
• Adds a new-release.py script that consolidates all next-release changes into a permanent .changes/x.y.z.json file.
• Adds a render.py script that re-renders a package's CHANGELOG.md file using all the .changes/<version>.json entries.
• Backfills all existing changelog entries for each package and re-renders each changelog file.

---

Workflow Eample

1. Generate a new changelog entry:

./scripts/changelog/new-entry.py -p <package_name> -t <type> -d <description>

This generates a file like<package_name>/.changes/next-release/<unique_id>.json with the following content:

{
  "type": "<type>",
  "description": "<description>"
}

2. Generate a new release

./scripts/changelog/new-release.py -p <package_name> -v <version>

This generates a new file like <package_name>/.changes/<version>.json with all changes to be released:

{
  "changes": [
    {
      "type": "<type>",
      "description": "<description>"
    },
    {
      "type": "<type>",
      "description": "<description>"
    }
  ]
}

3. Render the new changelog:

./scripts/changelog/render.py -p <package_name> -o <path_to_changelog>

This command will re-render the changelog and replace the content at <path_to_changelog>

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[arandito]

Looks good! Comments are mostly typos in the changelog and some minor nits.