feat: add support for Transit Gateway DLaaS

[sarah-ulmer]

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Relates OR Closes #0000

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccIBMTransitGatewayConnection_basic'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccIBMTransitGatewayConnection_basic -timeout 700m
?   	github.com/IBM-Cloud/terraform-provider-ibm	[no test files]
=== RUN   TestAccIBMTransitGatewayConnection_basic
2021/09/14 13:31:16 Inside testAccCheckIBMTransitGatewayConnectionExists :  
--- PASS: TestAccIBMTransitGatewayConnection_basic (287.21s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm	289.093s
testing: warning: no tests to run
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/internal/hashcode	(cached) [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/internal/mutexkv	(cached) [no tests to run]
?   	github.com/IBM-Cloud/terraform-provider-ibm/version	[no test files]
...

[sarah-ulmer]

@hkantare Can you please review and merge this?

[kennburger]

bump in case you missed this one... @hkantare please review