feat: Add support for bootstrapping feature toggles

[ivarconr]

Improve resilience by implementing the Bootstrap feature of Unleash. Feature write-up:
https://www.getunleash.io/blog/bootstrap-unleash-java-sdk-to-improve-the-resilience

Reference implementation in Java SDK:
Unleash/unleash-client-java#131

We need to bootstrap to be adapted to what makes sense in PHP. It should at least allow:

• manual passing of "json string of the full feature toggle configuration",
• loading from file
• loading from URL.

If the user want to bootstrap from a URL we should probably try to load the bootstrap content first, then the actual Unleash API, as it assumed to be faster than the Unleash API. I am also unsure how bootstrap and caching in the PHP should play together.