chore: allow manual initialization of the SDK

[marandaneto]

:bulb: Motivation and Context

Closes https://github.com/PostHog/posthog-flutter/issues/22 Closes https://github.com/PostHog/posthog-flutter/pull/113 Required for https://github.com/PostHog/posthog-flutter/pull/116

Automatic mode is still default so not a breaking change.

:green_heart: How did you test it?

Running auto init and automatic mode on android and ios

:pencil: Checklist

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

[marandaneto]

@thisames if you'd like to review as well :)

[thisames]

@thisames if you'd like to review as well :)

for me, everything is ok