added support for created_at and href support in is_ssh_key

[uibm]

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Relates OR Closes #0000

Output from acceptance testing:

% make testacc TEST=./ibm/service/vpc TESTARGS='-run=TestAccIBMISSSHKey_CreatedAtHref'
=== RUN   TestAccIBMISSSHKey_CreatedAtHref
--- PASS: TestAccIBMISSSHKey_CreatedAtHref (36.03s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     38.097s

% make testacc TEST=./ibm/service/vpc TESTARGS='-run=TestAccIBMISSSHKeyDatasource_CreatedAtHref'
=== RUN   TestAccIBMISSSHKeyDatasource_CreatedAtHref
--- PASS: TestAccIBMISSSHKeyDatasource_CreatedAtHref (47.39s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     49.427s