Added support for identifier in ibm_resource_instance

[ujjwal-ibm]

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Relates OR Closes #0000

Output from acceptance testing:

=== RUN   TestAccIBMResourceInstanceDataSource_identifier
--- PASS: TestAccIBMResourceInstanceDataSource_identifier (63.86s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/resourcecontroller      78.275s