Flagsmith instance clonning

[misakstvanu]

Hi,
clonning the instance with every configuration change is very expensive and not necessary in context of this library, I propose just setting the property and returning original object.

This also fixes Flagsmith.php line 183

    public function withCache(?CacheInterface $cache): self
    {
        if (is_null($cache)) {
            $this->with('cache', null); <-- result of this expression is not used anywhere (someone probably missed the clonning)
        }

        return $this->with(
            'cache',
            new Cache($cache, $this->cachePrefix, $this->timeToLive)
        );
    }

[matthewelwell]

I'm not sure why CI hasn't run on this PR but I'm definitely keen to see the tests run given this is a change to quite a fundamental part of the library. Can you make sure that you're up to date with the latest from the main branch in the target repository?

[misakstvanu]

I synced the repos.