First step to enable use of TF_REATTACH_PROVIDERS with PSS

[SarahFrench]

We kinda need the ability to use TF_REATTACH_PROVIDERS during development of PSS to work out issues with the protocol, so I've implemented the ability to use it with PSS in this branch, specifically this commit e84e15b. We're probably going to allow users to use TF_REATTACH_PROVIDERS with PSS in the end, too.

When creating the backend state file, to record how PSS is being used, this method will be used to determine if the provider is reattached and therefore needs special handling.

This PR:

• Adds test coverage for the preexisting parseReattachProviders function that parses TF_REATTACH_PROVIDERS and allows that ENV to be used.
• Adds a new function that's inspired by the function above, that just checks if a given provider is being supplied via TF_REATTACH_PROVIDERS.
• Moves reattach-related code into its own package

Target Release

N/A

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.

[radeksimko]

LGTM aside from that one question

[radeksimko]

Wouldn't it be easier to test things and reuse the function if we continued passing the value in as an argument, rather than read the environment variable?

[SarahFrench]

Yeah sure, we can avoid use of t.Setenv in tests that way. My rationalisation was that moving the call to os.Getenv into this package meant that the "TF_REATTACH_PROVIDERS" string wasn't floating around the codebase, but I can address that by making calling code use the new constant.

[SarahFrench]

I've refactored the function signature to require the new parameters in 12f8da6

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