fix: add plan type validation for public plan proposal

[dongsam]

Description

closes: #219

Tasks

• [x] add validation logic to delete, modify handler
• [x] add test case for the validations
• [x] gofmt, go 1.17 build

---

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• [ ] Code follows the module structure standards.
• [ ] Wrote unit and integration
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• [ ] Added relevant godoc comments.
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[dongsam]

@hallazzang @kogisin I know it's hard to review due to the retreat, but Let me merge it first due to the release schedule. Please review it later.

[jaybxyz]

LGTM as well