allow anonymous uuid generation to be customizable

[jimexist]

💡 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
• allow anonymous id generation to be configurable posthog-ios#133

💚 How did you test it?

unit test

📝 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]

@jimexist thanks for the PR.
I left 2 more comments, please add a https://github.com/PostHog/posthog-android/blob/main/CHANGELOG.md#next entry under the Next section.

[marandaneto]

@jimexist run make api and commit/push the changes, I tried to do it myself but jimexist/main (permission denied).