Backport of Use enumer to generate String() methods for most enums into release/1.16.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #25705 to be assessed for backporting due to the inclusion of the label backport/1.16.x.

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The person who merged in the original PR is:
@swenson
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

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

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We have many hand-written String() methods (and similar) for enums. These require more maintenance and are more error-prone than using automatically generated methods. In addition, the auto-generated versions can be more efficient.

Here, we switch to using https://github.com/loggerhead/enumer, itself a fork of https://github.com/diegostamigni/enumer, no longer maintained, and a fork of the mostly standard tool
https://pkg.go.dev/golang.org/x/tools/cmd/stringer. We use this fork of enumer for Go 1.20+ compatibility and because we require the -transform flag to be able to generate constants that match our current code base.

Some enums were not targeted for this change:

• Bit sets, which are not supported by enumer or stringer.
• Enums with non-standard naming schemes, like having a common suffix instead of a prefix, or using spaces instead of underscores or dashes.
• Enums that don't have a type associated

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Overview of commits

• 961bf20

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CI Results:
All Go tests succeeded! ✅

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[swenson]

Thanks! I'll go ahead and merge this for now.