Make the feature flag fetch optional

[markharding]

Currently, when a personalAPIKey is passed, it will immediately try and fetch the feature flags. As there is no way to provide an initial list of ->featureFlags (the constructor sets to an empty array), it makes it difficult to maintain and hydrate a local cache when using Local Evaluation.

This change it pretty simple and should have no breaking changes.

[neilkakkar]

Thanks @markharding for the PR! Trying to understand this a bit better, is your usecase that you have a separate cache of flags, and you want to use that to populate local eval flags? Perhaps because you're initialising the SDK per request?

[markharding]

Thanks @markharding for the PR! Trying to understand this a bit better, is your usecase that you have a separate cache of flags, and you want to use that to populate local eval flags? Perhaps because you're initialising the SDK per request?

Kind of, I couldn't see a way of using a cache at all with this SDK, as it the constructor rebuilds on each run. If you're using something like PHP-FPM, the class would be constructed on each user request. I'm also not really a huge fan of constructors doing anything more than handling dependencies. On every new PostHog\Client() call, this will always make a call to fetch the feature flags. My PR provides a way to bypass that logic so you can call ->loadFlags() yourself.

[neilkakkar]

Reasonable, sounds good 👍