feat: integrate feature flag poller to PostHog client (2/4)

[miguelhrocha]

This PR is meant to be reviewed after: #54

This change uses the changes introduced by #54 and adds them to the PostHog.java main file.

Notable changes

• The HttpSender was renamed to HttpInteractor. It felt weird to have a class named Sender that also gets. This class implements both the Sender interface and the Getter interface.
• The PostHog client will automatically start the featureFlagPoller if a personalApiToken is provided.

Not-implemented

Because time is limited, this is what needs to be implemented as a follow-up:

• The PostHog client should send FeatureFlag events.
• The PostHog client should have a customizable flag to turn on and off the local evaluation.