feat: add get session id

[GabrielRStabile]

💡 Motivation and Context

Resolve #133

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

[marandaneto]

Thanks @GabrielRStabile
Mind adding an entry to https://github.com/PostHog/posthog-flutter/blob/main/CHANGELOG.md ?
I will test, merge, and release it by the EoW.

[GabrielRStabile]

Thanks @GabrielRStabile Mind adding an entry to https://github.com/PostHog/posthog-flutter/blob/main/CHANGELOG.md ? I will test, merge, and release it by the EoW.

Done!

[abdallaabozaid]

@marandaneto can you please merge this if it's all done

[marandaneto]

@marandaneto can you please merge this if it's all done

I pushed 2 commits to your branch since there were missing things

[marandaneto]

Thanks @GabrielRStabile