Issue #3594111: Add the owner-passphrase feature to the roadmap

Frank Mablyrequested to merge
issue/pdv-3594111:3594111-owner-passphrase into 1.x

Adds an Owner passphrase section to docs/roadmap.md (gate vault content with a user-held secret; protect against the operator). Docs-only. See https://www.drupal.org/project/pdv/issues/3594111 .

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