fix: move to Blocked state when HugePages are not available

[dariofaccin]

Description

This PR adds two validations to the UPF charm when enabling HugePages: the first is the support of the required CPU flags (pdpe1gb); the second is the capacity of enough HugePages (2Gi) on the K8S nodes. If the free HugePages (allocatable minus allocated) are not enough, the charm will remain in Waiting status.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library

[Gmerold]

PR description says:
(...) If the free HugePages (allocatable minus allocated) are not enough, the charm will remain in Waiting status.
The code only checks the capacity tho. I think it would be great to have the feature implemented as described above.