allow anonymous id generation to be configurable

[jimexist]

:bulb: Motivation and Context

this makes it aligned with the js/ts sdk

• https://github.com/PostHog/posthog-js/blob/55694356e168568700f3370f26d636cbe09bf795/src/posthog-core.ts#L169
• https://github.com/PostHog/posthog-android/pull/132

:green_heart: How did you test it?

swift test

:pencil: 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 for the PR @jimexist Could you add a test to validate the behavior? PostHogSessionManagerTest would be the file I think

[marandaneto]

For the lint issue: https://github.com/PostHog/posthog-ios/pull/134