feat: feature flag polling for local evaluation (1/4)

[miguelhrocha]

Hello!

When I was looking on how to contribute to the PostHog's roadmap, I've found that one of the top items is implementing Feature Flags in Java for local evaluation. See issue: PostHog/posthog#16419

The FeatureFlagPoller is set to fetch for new feature flags every 5 minutes. The poller also provides a forceReload method in case the consumer wants to have an up-to-date view of the feature flags.

I based most of my implementation from https://github.com/PostHog/posthog-go

I wanted to get an understanding on how to interact with PostHog's product, and Feature Flags seems a straightforward entrypoint to the portfolio.

Review order

This is the first of two PRs. I wanted to keep the scope of the change simple to facilitate the review. This PR is mainly POJOs; FeatureFlagPoller being the exception.

1. feat: integrate feature flag poller to PostHog client (2/4) #55: use the poller implemented here in the PostHog client.
2. feat: send feature flag event to product analytics (3/4) #56: send feature flag events to product analytics every time a consumer calls a feature flag method.
3. feat: get all feature flags related to a distinct_id (4/4) #57: get all the feature flags associated to a distinct_id

The pull-requests mentioned above should not be reviewed until the dependencies are merged.

[neilkakkar]

Hey @miguelhrocha - thanks a lot for your PRs! Even though, as @marandaneto said this probably wouldn't make it in right now, I'd love to send you a posthog merch code because of the effort - this is great, love to see it :)

Send me an email please at neil @ posthog . com :)