Add backend_url option to client initializer

[ludwigbacklund]

To use a custom backend URL, for example to be able to use the EU Novu API, you can pass a custom backend_url when initializing the client.

client = Novu::Client.new(
  access_token: '<your-novu-api_key>', 
  backend_url: 'https://eu.api.novu.co/v1'
)

[unicodeveloper]

Thanks for submitting this PR @ludwigbacklund

@Eazybright Please can you help review this PR?

[Augustalex]

I also have this issue. Would be great if we can have this merged 🙏

[Eazybright]

@unicodeveloper the PR is all good

[unicodeveloper]

@ludwigbacklund Tests are failing. https://github.com/novuhq/novu-ruby/actions/runs/8188109840/job/22390018410#step:6:1294

Please can you send a PR to test this and ensure all the tests are passing. cc @Eazybright

[ludwigbacklund]

@unicodeveloper #74

[unicodeveloper]

@ludwigbacklund @Augustalex New release is live. Update your packages to v2.0.0