Add `bgp_config` attribute to `tier0_gateway` data source

[ens-scmeeu]

Is your feature request related to a problem? Please describe.

We need to reference the bgp_config attribute on a tier0_gateway in a different Terraform repo, but currently this attribute is not exposed in the data source

Describe the solution you'd like

We would like the bgp_config attribute added to the existing data source

Describe alternatives you've considered

Currently we use terraform_remote_state to retrieve the bgp_config attribute. It works, but it is a bit kludgy and we would ideally like independence from the state file.

Additional context

No response