Feat: non-destructive increase of a storage volume size

[NamelessOne91]

Related to #952 and #1017

This PR adds support for a non-destructive resize when a storage volume size is increased.
Shrinking a storage volume will still ultimately result in destroying the existing one and recreating it.

It would have been possible to handle both cases, shrinking and expanding, in the UpdateContext function, but having a CustomizeDiff function allows to correctly differentiate between a resource update and a destroy + add, notifying it to the user.

[scabala]

Thank you for this PR @NamelessOne91!

I'll try to test it in following days and give my feedback.

[dmacvicar]

ℹ️ ℹ️ ℹ️ ℹ️ ℹ️ ℹ️

This contribution is relevant to the legacy version of the provider, which is now in the v0.8 branch of this repository.

Future development is based on a new provider, which is not compatible with this one, nor does share code.

As the new provider solves many issues with the legacy, and to make development in my free time more enjoyable, I have decided to close all PRs for the legacy provider, and to ask to check if the contribution would apply to the new one. This also to encourage trying the new version.

• https://registry.terraform.io/providers/dmacvicar/libvirt/latest

and check the documentation:

• https://github.com/dmacvicar/terraform-provider-libvirt/blob/main/README.md
• https://registry.terraform.io/providers/dmacvicar/libvirt/latest/docs

You are free to reopen the PR for v0.8, which is targetted now to the v0.8 branch. We may also start a discussion if we can assemble a team of maintainters for the legacy branch. My efforts will go into the new provider.

I ask you to check the new provider and re-evaluate this contribution. 🙏