secret: Add Fallback stash

[abhinav]

Adds a secret storage that will allow a primary and secondary store. The secondary store is used only if the primary is broken for non-expected reasons.

Notably, LoadSecret returning ErrNotFound is not an unexpected breakage.

[abhinav]

This change is part of the following stack:

• 297 ◀

  • 298

_{Change managed by git-spice.}