Add setAccessToken()

[Olen]

Nice project.

I just added a function to the tesla class to setAccessToken.

public function setAccessToken(string $accessToken)
{   
    $this->accessToken = $accessToken;
}

It just makes the coding easier, as you can always init without a token, and if you have a cached token, you can add it later in the process

[stephangroen]

Thank you. You can submit a PR if you'd like.