feat: support speed, stability and similarity_boost in TtsOverrides

[Darnasis]

Problem

conversation_config_override.tts accepts voice_id, speed, stability and similarity_boost, and agents expose a Security → Overrides toggle for each one. The Kotlin SDK can only express voice_id:

data class TtsOverrides(
    val voiceId: String? = null
)

ovr.tts?.voiceId?.let { ttsObj.put("voice_id", it) }

So on Android the other three are unreachable even with the toggles enabled — there is no way to start a conversation at a different speed. The Python/JS SDKs and the docs' own override examples all support the full block, so this reads as an Android gap rather than an intentional limit.

Concretely: we build a voice tutor whose users set a speech-rate preference. We can apply their chosen voice, but not their chosen speed, purely because the field cannot be expressed.

Change

• TtsOverrides gains speed, stability and similarityBoost, all nullable, defaulting to null — source- and binary-compatible for existing callers, who keep constructing TtsOverrides(voiceId = ...) unchanged.
• ConversationOverridesBuilder serializes them with the documented snake_case keys, keeping the existing omit-when-null behaviour so a partially-populated TtsOverrides never widens into explicit nulls (the docs specifically ask callers to omit rather than send nulls).

Tests

Adds ConversationOverridesBuilderTest — as far as I can tell the first coverage of constructOverrides:

• each field serializes, with correct similarity_boost snake_case mapping
• all four together
• omit-when-null: a TtsOverrides(speed = 1.1) emits exactly one key
• an entirely empty TtsOverrides emits no tts object at all (existing behaviour, now pinned)
• agent overrides still serialize alongside tts

org.json was already a testImplementation dependency, so no build changes were needed.

./gradlew :elevenlabs-sdk:testDebugUnitTest passes (7/7 new, existing suite green).

Notes

Happy to adjust naming (similarityBoost follows the existing camelCase-in-Kotlin / snake_case-on-the-wire convention used by voiceId and firstMessage) or to narrow the change to speed alone if you'd prefer a smaller surface.