expose FabricClient interface

[jordanstephens]

Description

This PR extracts a mockable FabricClient interface to support unit testing in the V1 Defang Pulumi Provider: DefangLabs/pulumi-defang#54

In order to do this, I needed to avoid accessing data attributes directly, so I implemented GetTenantName and GetController.

Linked Issues

DefangLabs/pulumi-defang#54

Checklist

• I have performed a self-review of my code
• I have added appropriate tests
• I have updated the Defang CLI docs and/or README to reflect my changes, if necessary

[lionello]

The gRPC interface is perfect for mocking, so I'd mock that instead.

[jordanstephens]

The gRPC interface is perfect for mocking, so I'd mock that instead.

GrpcClient is a type which implements the FabricClient interface. My understanding is that if you want to mock a type in go, you define an interface which that type implements, then you create a new type which also implements interface. That's what I've done here.