Add vcn > display_name to oci_core_subnet.

[rajlearner17]

Is your feature request related to a problem? Please describe.
Same as #137

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[bigdatasourav]

As discussed , currently there is no parent child relationship present for subnet api with vcn table. To have vcn_name in subnet table we need to do a separate api call which is a expensive option.

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'This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 30 days.'

[github-actions]

'This issue was closed because it has been stalled for 90 days with no activity.'