chore: SDK API client improvements

[maxtwardowski]

This PR introduces the following changes:

• Embed SDK's ClientInterface into ClientWithResponsesInterface
• Use ClientWithResponsesInterface interface in ProviderConfig instead of concrete *sdk.ClientWithResponses type

This makes testing easier as we're no longer limited to response-less API calls, for example:

ctrl := gomock.NewController(t)
mockClient := mock_sdk.NewMockClientWithResponsesInterface(ctrl)
provider := &ProviderConfig{
    api: &sdk.ClientWithResponses{
        ClientInterface: mockClient,
    },
}

// Calling EXPECT() is now possible for both WithResponse and "response-less" methods
mockClient.EXPECT().UsersAPIListOrganizations(gomock.Any())
mockClient.EXPECT().UsersAPIListOrganizationsWithResponse(gomock.Any())

provider.api.UsersAPIListOrganizations(ctx)
provider.api.UsersAPIListOrganizationsWithResponse(ctx)