Feature request: support JWT-enabled API keys

[lazytesting]

Iterable supports to use JWT enabled API keys (see https://support.iterable.com/hc/en-us/articles/360050801231#using-iterable-s-mobile-sdks-with-jwt-enabled-api-keys). This to prevent that an attacker could misuse the api key (see also https://support.iterable.com/hc/en-us/articles/360043464871-API-Keys-#jwt-authentication).

It would be great if this Flutter SDK would also support the usage of JWT enabled api keys.

[cdmunoz]

Thanks @lazytesting
Feel free to help us with a PR with this feature! 😃