Feature request: Add Kakao OAuth2 provider support

### Prerequisites

- [x] I have written a descriptive issue title
- [x] I have searched existing issues to ensure the feature has not already been requested

### 🚀 Feature Proposal

## Description
`@fastify/oauth2` does not include support for Kakao OAuth2.

Kakao Login is one of the most widely used OAuth providers in South Korea.

- KakaoTalk has over **100M+ downloads on Google Play**
- Kakao accounts are commonly used as a primary login method for consumer services

## Proposal
- Add a built-in `kakao` provider following existing provider patterns

## Willingness to contribute
I am willing to implement this feature and submit a PR if maintainers agree with the approach.

### Motivation

_No response_

### Example

_No response_