Add webhook validation for transit gateway's global routing field

[dharaneeshvrd]

Extend existing conditions to check valid PowerVS zone and VPC region is used

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Fixes #1826

Special notes for your reviewer:

/area provider/ibmcloud

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

Add webhook validation for transit gateway's global routing field

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[dharaneeshvrd]

/cc @Karthik-K-N

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dharaneeshvrd, Prajyot-Parab

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