Update data stored in plan files to enable using PSS with saved plans

[SarahFrench]

Some prototyping/implementation for the PSS project that's only relevant to the apply command.

These changes enable saving PSS config in a plan, similar to how a planfile can store backend config and be used to configure a backend.Backend during an apply command that is supplied with a pre-existing plan file.

Target Release

N/A

Rollback Plan

• [x] If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• [ ] This change is user-facing and I added a changelog entry.
• [x] This change is not user-facing.