feat(is): support for endpoint gateway resource bindings

[uibm]

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Relates OR Closes #0000

Output from acceptance testing:

=== RUN   TestAccIBMIsVirtualEndpointGatewayResourceBindingBasic
--- PASS: TestAccIBMIsVirtualEndpointGatewayResourceBindingBasic (171.26s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     173.867s

=== RUN   TestAccIBMISVirtualEndpointGateway_BasicConfig
--- PASS: TestAccIBMISVirtualEndpointGateway_BasicConfig (83.81s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     86.373s

=== RUN   TestAccIBMIsVirtualEndpointGatewayResourceBindingsDataSourceBasic
--- PASS: TestAccIBMIsVirtualEndpointGatewayResourceBindingsDataSourceBasic (133.07s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     135.640s

=== RUN   TestAccIBMIsVirtualEndpointGatewayResourceBindingDataSourceBasic
--- PASS: TestAccIBMIsVirtualEndpointGatewayResourceBindingDataSourceBasic (127.97s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     130.008s

=== RUN   TestAccIBMISVirtualEndpointGateway_DnsResolutionBindingMode
--- PASS: TestAccIBMISVirtualEndpointGateway_DnsResolutionBindingMode (118.53s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     121.031s

=== RUN   TestAccIBMISVirtualEndpointGateway_DnsResolutionBindingMode
--- PASS: TestAccIBMISVirtualEndpointGateway_DnsResolutionBindingMode (870.38s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     872.923s