fix: add plan type validation for public plan proposal

[dongsam]

Description

closes: #219

Tasks

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

---

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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.

[hallazzang]

LGTM

[jaybxyz]

LGTM as well