Support input vars on Waypoint add-ons

[paladin-devops]

Changes proposed in this PR:

Enable app developers to set variables when installing an add-on.

How I've tested this PR:

I created an add-on for an application with an input variable using the CLI.

How I expect reviewers to test this PR:

Create an add-on for an application with an input variable using the CLI.

Checklist:

• Tests added if applicable
• CHANGELOG entry added or label 'pr/no-changelog' added to PR

  Run CHANGELOG_PR=<PR number> make changelog/new-entry for guidance
  in authoring a changelog entry, and commit the resulting file, which should
  have a name matching your PR number. Entries should use imperative present
  tense (e.g. Add support for...)