Add OnDialogueStart / OnDialogueStop to DialogueRunner

[nrvllrgrs]

• Please check if the pull request fulfills these requirements

• [x] Tests for the changes have been added (for bug fixes / features)
  • [x] Does it pass all existing unit tests without modification?
    • If not, what did you change?
    • If you altered it significantly, what coverage issue did you fix?
• [x] Docs have been added / updated (for bug fixes / features)
• [x] CHANGELOG.md has been updated to describe this change

• What kind of change does this pull request introduce?

• [x] Feature

• What is the current behavior?

• What is the new behavior (if this is a feature change)? Added OnDialogueStart and OnDialogueStop as UnityEvent to DialogueRunner

• Does this pull request introduce a breaking change? May introduce breaking change. Changed set_IsDialogueRunning from public to private. Enforces users to use StartDialogue and Stop methods.

• Other information: