PSS: Ensure experimental backend codepath is isolated + gated correctly

[radeksimko]

This just moves the code that was added to init.go since 7f29df9

https://github.com/hashicorp/terraform/commits/main/internal/command/init.go

The goal is to better isolate the experimental codepath and reduce the risk of breaking things in the non-experimental ones.

Additionally, the PR also updates the condition and diagnostic wording to ensure it is gated not just behind experimental build but also behind the feature CLI flag.

I ran into a somewhat confusing/misleading diagnostic myself on init which motivated this PR:

Initializing the state store...
╷
│ Error: Provider unavailable
│ 
│   on main.tf line 7, in terraform:
│    7:   state_store "pss_fs" {
│ 
│ The provider pss ("registry.terraform.io/hashicorp/pss") is required to initialize the "pss_fs" state
│ store, but the matching provider factory is missing. This is a bug in Terraform and should be reported.
╵

Target Release

1.15.x

Rollback Plan

• 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.
• This change is not user-facing.