Backport of keyring: support external KMS for key encryption key (KEK) into release/1.8.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #23580 to be assessed for backporting due to the inclusion of the label backport/1.8.x.

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

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In Nomad 1.4.0, we shipped support for encrypted Variables and signed Workload Identities, but the key material is protected only by a AEAD encrypting the KEK. Add support for Vault transit encryption and external KMS from major cloud providers. The servers call out to the external service to decrypt each key in the on-disk keystore.

Ref: https://hashicorp.atlassian.net/browse/NET-10334
Fixes: #14852

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Notes for reviewers:

• This PR is unfortunately large, but most of it is config parsing, which is irreducibly big 😿
• I've pulled out documentation PR here to keep review size manageable: docs: external KMS configuration #23600
• I'm working on E2E PR here which will also cover the new rotation work: keyring: E2E testing for KMS/rotation #23601

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Overview of commits

• c970d22

[hashicorp-cla-app]

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