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## What type of PR is this?

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/kind feature
## What this PR does / why we need it:
This PR adds support for database accessory which defines the relational
database service (rds) provided by a specified cloud vendor.
## Which issue(s) this PR fixes:

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## Special notes for your reviewer:
The implementation of Database Generator can be found at [feat: add
database generator](KusionStack/kusion#435)

### Does this PR introduce a user-facing change?

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### Additional documentation e.g., design docs, usage docs, etc.:

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