feat(atracker): support for managed_by property in iac.

[fidel-ibm]

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Relates OR Closes #0000

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'

➜  terraform-provider-ibm git:(fidel-ruiz/feat-iac-managed-by) ✗ make testacc TEST=./ibm/service/atracker TESTARGS='-run=TestAccIBMAtrackerTarget*'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./ibm/service/atracker -v -run=TestAccIBMAtrackerTarget* -timeout 700m
=== RUN   TestAccIBMAtrackerTargetsDataSourceBasic
--- PASS: TestAccIBMAtrackerTargetsDataSourceBasic (15.07s)
=== RUN   TestAccIBMAtrackerTargetsDataSourceAllArgs
--- PASS: TestAccIBMAtrackerTargetsDataSourceAllArgs (15.56s)
=== RUN   TestAccIBMAtrackerTargetBasic
--- PASS: TestAccIBMAtrackerTargetBasic (19.11s)
=== RUN   TestAccIBMAtrackerTargetAllArgs
--- PASS: TestAccIBMAtrackerTargetAllArgs (23.03s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/atracker        74.407s