models: add Web App BottomButton type with iconCustomEmojiId support

[luchojuarez]

Summary

This PR adds support for Telegram Web Apps BottomButton in the models package.

What’s included

• Added models.BottomButton in models/web_app.go
• Included all documented fields for the Web Apps object:
  • type
  • iconCustomEmojiId
  • text
  • color
  • textColor
  • isVisible
  • isActive
  • hasShineEffect
  • position
  • isProgressVisible
• Added models/web_app_test.go with TestBottomButtonJSON to verify JSON unmarshal/marshal behavior and round-trip consistency

Why

Telegram Bot/Web App updates introduced iconCustomEmojiId for BottomButton.
This model makes it easier for users who integrate Mini App payloads in Go to parse and serialize button state with correct field names.

Test plan

• go test ./models/ -run TestBottomButtonJSON
• go test ./...