Add selector_mode field to chains

[LucasPickering]

Description

Describe the change. If there is an associated issue, please include the issue link (e.g. "Closes #xxx"). For UI changes, please also include screenshots.

This opens up more options for handling selector results. It also allows us to simplify the dynamic select chain input process, to just a single template.

Known Risks

What issues could potentially go wrong with this change? Is it a breaking change? What have you done to mitigate any potential risks?

Added complexity for the user, potentially unintuitive behavior for the new default mode of auto.

QA

How did you test this?

Unit tests, updated the stock slumber.yml and tested that

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Only user-facing changes belong in the changelog. Internal changes such as refactors should only be included if they'll impact users, e.g. via performance improvement.
• Did you remove all TODOs?
  • If there are unresolved issues, please open a follow-on issue and link to it in a comment so future work can be tracked

[LucasPickering]

cc @anussel5559 I changed the way dynamic selects work, you no longer need the !dynamic tag and it's just a single template now, no selector field. Instead I extended Chain.selector to support different modes of handling single vs multiple values, which makes it more powerful in general. If you're using the master branch for dynamic selects you'll need to update your collections.