Added support for snapshot service tags

[ujjwal-ibm]

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Relates OR Closes #0000

Output from acceptance testing:

% make testacc TEST=./ibm/service/vpc TESTARGS='-run=TestAccIBMISSnapshot_serviceTags'
=== RUN   TestAccIBMISSnapshot_serviceTags
--- PASS: TestAccIBMISSnapshot_serviceTags (262.35s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     264.318s

% make testacc TEST=./ibm/service/vpc TESTARGS='-run=TestAccIBMISSnapshotsDatasource_serviceTags'
=== RUN   TestAccIBMISSnapshotsDatasource_serviceTags
--- PASS: TestAccIBMISSnapshotsDatasource_serviceTags (245.55s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     247.170s

% make testacc TEST=./ibm/service/vpc TESTARGS='-run=TestAccIBMISSnapshotDatasource_serviceTags'
=== RUN   TestAccIBMISSnapshotDatasource_serviceTags
--- PASS: TestAccIBMISSnapshotDatasource_serviceTags (193.30s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     194.908s