feat: Add possibility to disable polling

[chriswk]

So, there are situations where you don't necessarily want to poll, so taking inspiration from our already existing disableMetrics() flag on UnleashConfig, this PR adds a disablePolling() flag which stops the repository from adding a task to our scheduled executor.

So if you don't use synchronousFetchOnInitialization() and have no bootstrap, it is possible to have a situation where you don't have any features.

If you want to fetch once and then stop, set fetchTogglesInterval to 0, that will cause the executor to be set to runOnce instead of at a fixedRate

[stale[bot]]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[thomasheartman]

@chriswk @sighphyre What's the state here? Doesn't seem like anything's happened for a while. Is this still being worked on?