Add tags collector exposing vSphere tag assignments for VMs

[poulpreben]

What

Adds a new tags collector that exposes vSphere tag assignments for virtual machines, so tag/category metadata can be joined onto the existing VM metrics in PromQL.

For every tag attached to a VM it emits:

vmware_vm_tag{category="environment",tag="production",vcenter="vcenter01.example.com",vm="app-server-01",vmmo="vm-1001"} 1
vmware_vm_tag{category="environment",tag="staging",vcenter="vcenter01.example.com",vm="app-server-02",vmmo="vm-1002"} 1
vmware_vm_tag{category="backup-policy",tag="no-backup",vcenter="vcenter01.example.com",vm="db-server-01",vmmo="vm-1003"} 1

which joins onto any other VM metric via the vmmo label, e.g.:

vmware_vm_cpu_usagemhz_average
  * on(vmmo) group_left(category, tag)
    vmware_vm_tag

How

• vmware/collectors/tags.go (new): registers a tags collector (flag -collector.tags, enabled by default, honors -disable.default.collectors). It fetches all VM refs/names through the property collector (name only, so the property fetch stays cheap), resolves tag categories once per scrape, then calls GetAttachedTagsOnObjects in batches of 500 VMs per vAPI request.
• vmware/api/vmware.go: tags live behind the vAPI REST endpoint, which needs its own session on top of the SOAP one. Login now also creates the REST session and stores a tags.Manager in loginData - but only when the tags collector is enabled, so nothing changes for users who disable it. Logout tears the REST session down.
• README.md: documents the new flag and metric.

No new dependencies - only the vapi/rest and vapi/tags packages from the already-vendored govmomi version.

Testing

• go build ./..., go vet ./... and the existing test suite pass.
• Verified against a live vCenter 8 instance: the collector returned all tag assignments (metric count matched the number of tag attachments in the inventory), vmware_scrape_collector_success{collector="tags"} 1, and the collector completed in ~0.35s for an inventory of several hundred VMs - comparable to the vm collector on the same scrape.

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