feat(cluster): add proxmox_virtual_environment_metrics_server resource

[rafsaf]

Contributor's Note

• I have added / updated documentation in /docs for any user-facing features or additions.
• I have added / updated acceptance tests in /fwprovider/tests for any new or updated resources / data sources.
• I have ran make example to verify that the change works as expected.

Proof of Work

Hi, this adds new metrics server resource and datasource to the provider. With somewhat improved language skills after PRs in last year, i think I probably got everything right more than less. It is opinionated to prefix graphite and influxdb specific variables in resource but leave it as-is in API client, but felt right to me.

Take your time for that... I got examples and acc tests working fine to help make it happen as much as possible.

PS. Seems Proxmox folks made not-broken-and-weird API responses this time for metrics server, which means boolean in docs is either 1 or 0 hence I used int64 in client, usually it required types.CustomBool since there were strings "1" or "0" if I remember correctly 😃

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Closes #0000 | Relates #0000