Add dbcrypt support for user secret values. When database encryption is
enabled, secret values are transparently encrypted on write and
decrypted on read through the existing dbcrypt store wrapper.
- Wrap `CreateUserSecret`, `GetUserSecretByUserIDAndName`,
`ListUserSecretsWithValues`, and `UpdateUserSecretByUserIDAndName` in
enterprise/dbcrypt/dbcrypt.go.
- Add rotate and decrypt support for user secrets in
enterprise/dbcrypt/cliutil.go (`server dbcrypt rotate` and `server
dbcrypt decrypt`).
- Add internal tests covering encrypt-on-create, decrypt-on-read,
re-encrypt-on-update, and plaintext passthrough when no cipher is
configured.
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