feat: Add Kotlin type generator

[thagikura]

What kind of change does this PR introduce?

Feature

What is the current behavior?

Type generators exist for TypeScript, Swift, Go, and Python, but not for Kotlin. Kotlin developers
using the supabase-kt SDK must manually write
@Serializable data classes to match their database schema.

• [Feature request]: Generate Kotlin types from Database supabase-community/supabase-kt#647
• https://github.com/orgs/supabase/discussions/43955

What is the new behavior?

Adds a Kotlin type generator at GET /generators/kotlin that produces @Serializable data classes
compatible with kotlinx.serialization. Follows the
same pattern as the Swift generator (#779).

For each table, three data classes are generated (Select, Insert, Update). Views get Select
only. PostgreSQL enums become @Serializable enum class with @SerialName values.

Example output:

import kotlinx.serialization.SerialName
import kotlinx.serialization.Serializable

@Serializable
enum class UserStatus {
    @SerialName("ACTIVE") ACTIVE,
    @SerialName("INACTIVE") INACTIVE
}

@Serializable
data class UsersSelect(
    val id: Long,
    val name: String? = null,
    val status: UserStatus? = null
)

@Serializable
data class UsersInsert(
    val id: Long? = null,
    val name: String? = null,
    val status: UserStatus? = null
)

@Serializable
data class UsersUpdate(
    val id: Long? = null,
    val name: String? = null,
    val status: UserStatus? = null
)

Additional context

- @SerialName annotations are only added when the camelCase property name differs from the column name
- Kotlin reserved words (in, when, val, etc.) are escaped with backticks
- Nullability follows the same rules as Swift: Select respects column nullability, Insert makes
identity/generated/default columns nullable, Update makes everything nullable
- Inline snapshot test added covering tables, views, materialized views, enums, composite types, and
enum/table type references