feat: add support for fallback function

[ivarconr]

Today it is possible to define a fallback value which the IsEnabled will return if the feature toggle is not defined.

We want to also allow the user to send in a fallbackFunction instead of a fallback value (which will take precedence over fallback value).

The fallback function should get to arguments injected: toggleName and unleashContext.

Example on how it would work:

unleash.IsEnabled("foobar", unleash.WithFallbackFunction(myFunc))

If the user injects both a fallback value and fallbackFunction, like the example below, the result should be to only call the fallback function in the scenario that the toggle is not defined:

unleash.IsEnabled("foobar", unleash.WithFallback(true), unleash.WithFallbackFunction(myFunc))