[flagd-provider] add constructor with configuration object

[toddbaert]

The flagd provider currently supports getting config from env-vars, but we should also add the ability to configure it with a simple config object, as discussed here and here with @bacherfl and @odubajDT .

Configurations provided in this options object should override values from the env vars, as is the case in the java/js equivalent.

[odubajDT]

Hello, I would like to take this issue :)