Add environment property to context

[fenix20113]

Add the environment property to context to be able to:

1. Use the Constrain on a specific environment
2. Create custom strategies based on the environment.

[RikudouSage]

The current error is because the environment is not set in \Unleash\Client\Tests\ClientSpecificationTest::createContext(). That's why I would advise to do it similarly to how the hostname is handled in context:

    public function getHostname(): ?string
    {
        return $this->findContextValue(ContextField::HOSTNAME) ?? (gethostname() ?: null);
    }

    public function setHostname(?string $hostname): self
    {
        if ($hostname === null) {
            $this->removeCustomProperty(ContextField::HOSTNAME);
        } else {
            $this->setCustomProperty(ContextField::HOSTNAME, $hostname);
        }

        return $this;
    }

And in constructor you will just call $this->setEnvironment($environment) while removing the private modifier from the constructor parameter. But it's up to you, I'll accept both solutions.

[fenix20113]

Thanks for the explanation. I updated \Unleash\Client\Tests\ClientSpecificationTest::createContext()

[RikudouSage]

Thank you for your contribution!