Added service dropdown picker with null support to the platform_resource widget

[Khuzaima05]

Description

Enhanced the platform_resource widget to support service dropdowns picker.

issue: https://github.ibm.com/GoldenEye/issues/issues/16407

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Release notes content

• Added support for service dropdown picker in the platform_resource widget.
• The dropdown also includes a null option, allowing users to choose null.

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[ocofaigh]

@Khuzaima05 Any update on this PR? Did you test this is working? Can you maybe include screenshots of the tests in the PR?

[Khuzaima05]

@ocofaigh , I have added picker for these:

1. existing_subnet_ids
2. existing_secrets_manager_instance_crn
3. existing_vpc_crn
4. existing_cos_instance_crn

But as per this PR : #814, we have hide the most of these options in UI. In UI we can not see these options . I dont think it will make sense to add picker for these.