feat(misk-admin): introspect plain Kotlin data classes for web action form metadata

[kchung]

Description

The misk-admin web-actions tab renders a request body skeleton in the editor by walking the request type's fields and emitting a JSON template with empty defaults. Currently this only works when the @RequestBody parameter type is a Wire com.squareup.wire.Message. For plain Kotlin data class request types the editor falls back to {}, giving developers no clue what fields the endpoint accepts.

This PR generalizes the existing Wire-only support to also walk plain Kotlin data class request types using Kotlin reflection so generic type parameters like List<Foo> and Map<String, Bar> are first-class.

Scenario. Take an admin endpoint whose request body is a plain Kotlin data class — for example addBrandsToCohort(@RequestBody request: AddBrandsRequest) where:

data class AddBrandsRequest(val brands: List<BrandInput>)
data class BrandInput(val brandToken: String, val locationTokens: List<String>?)

When a developer opens the web-actions tab and selects this endpoint, the editor pre-fills a JSON skeleton.

Before (today, on master):

{}

After (this PR):

{
  "brands": []
}

The editor surfaces field names and type-based defaults at the same level of structural detail Wire users already get.

Scope and non-goals

• Strictly field discovery (parity with Wire). No new annotations, no semantic example values
• Map handling mirrors Wire's pre-existing behavior (key skipped, value emitted as repeated)

Testing Strategy

• added unit tests
• add integration tests
• tested it locally with exmplar

Checklist

• I have added tests to have confidence my changes work as expected.