feat: add support for setting user agent on event

[crleona]

Summary

Adds support for user_agent property

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?: no

[Copilot]

Pull Request Overview

Adds support for a new user_agent field on Event objects, enabling clients to set a custom user-agent string when sending events.

• Extend EventOptions property mapping to include user_agent
• Add user_agent parameter to EventOptions and all BaseEvent subclasses
• Update docstrings to describe the new parameter

Comments suppressed due to low confidence (2)

src/amplitude/event.py:167

• No unit tests were added to verify that the user_agent property is correctly included in the event payload. Consider adding tests for this behavior.

    "user_agent": ["user_agent", str]

src/amplitude/event.py:213

• Public documentation (e.g., README, reference guide) should be updated to document the new user_agent parameter and its usage examples.

        user_agent (str, optional): The user agent string.

[sojingle]

LGTM!