Backport of Generalize the idea of a "plan mode", vs just destroy flag into v0.15

[teamterraform]

Backport

This PR is auto-generated from #28489 to be assessed for backporting due to the inclusion of the label 0.15-backport.

The below text is copied from the body of the original PR.

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Previously there were only two planning modes: normal mode and destroy mode. In that context it made sense for these to be distinguished only by a boolean flag.

We're now getting ready to add our third mode, "refresh only". This establishes the idea that planning can be done in one of a number of mutually-exclusive "modes", which are related to but separate from the various other options that serve as modifiers for the plan operation.

This commit only introduces the new plans.Mode type and replaces the existing "destroy" flag fields with a field of that type. This doesn't cause any change in effective behavior because Terraform Core still supports only NormalMode and DestroyMode, with NewContext rejecting an attempt to create a RefreshMode context for now, and the CLI layer having no codepath that tries to set it.

It is in retrospect a little odd that the "destroy" flag was part of ContextOpts rather than just an argument to the Plan method, but refactoring that would be too invasive a change for right now so we'll leave this as a field of the context for now and save revisiting that for another day.

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For this PR I've just peeled off the first two commits from #28297. These two just change internal implementation details of how we represent destroy mode without adding any new functionality. I want to land these first and early because they are cross-cutting and thus likely to come into conflict with other work if they were part of a long-lived feature branch.

With these in place the remaining work should involve more localized commits that we can land without necessarily colliding with other work in other subsystems.

I've marked this for backport primarily because otherwise it seems likely that other unrelated backports to v0.15 will end up failing due to this being a cross-cutting change. However, it also seems likely that at least parts of the planned functionality for refresh-only plans will also land in the v0.15 branch, and so this backport will support that.

[apparentlymart]

I've backported this manually with git cherry-pick because the backport assistant generates poor commit messages and this is a multi-commit set and so I can't repair them using the squash UI as I typically do.

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