Command to import a specific BottleRocket template to vCenter based on EKS-A version

Some user needs to do customization on things like `diskGiB`, `memoryMiB`, on top of the default BottleRocket template EKS-A provides, before running cluster create/upgrade command. Some of those customizations need to be built and imported ahead of time instead of overriding them on the fly during cluster provisioning, as EKS-A default import adds snapshot in the template.

Also as mentioned in #5212, we lack documentation about which BR template should be imported for each EKS-A version, plus it can be difficult for user to need to go through the doc and find the right BR version for the EKS-A version, we suggest to have a standalone command to import a specific BR template for the EKS-A version to vCenter. So that from users perspective, they no longer need to know which BR template to download and import based on EKS-A release number, instead running the command shall do the trick.

To support the feature, we could extract the template import logic from existing code. Though some BR template can't be imported with `govc` and we will need to verify all templates.

Locate the existing BottleRocket template import logic and trace how EKS-A selects a template for an EKS-A version. Define the standalone command around that flow, verify which templates can be imported with govc, and confirm that the command imports the version-appropriate template into vCenter before cluster provisioning.

Written by the indexing model from the issue text.