[receiver/vcenter] Enable re-aggregation feature

[dmitryax]

Part of #45396.

Enable the re-aggregation feature for the vcenter receiver by:

1. Setting reaggregation_enabled: true in metadata.yaml
2. Ensuring all attributes are properly defined with appropriate levels:
   • required: cannot be reaggregated
   • recommended: enabled by default but can be disabled by users
   • opt_in: disabled by default but can be enabled by users
3. Validate the feature after the feature gate is enabled: all existing tests pass, manual check

[github-actions]

Pinging code owners:

• receiver/vcenter: @schmikei @ishleenk17

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[AdityaDokrimare]

@dmitryax , Can I work on it?

[hariom-hp]

Hi, I would like to work on this issue. Could you assign it to me?