[vsphere] Create new package for better managing VMWare sessions

*USER STORY:*

This story can apply to an OpenShift engineer, VMWare engineer, or community developer:

As an X engineer, I want to have session caching and management handled by govmomi so that I don't have to copy session management code or reimplement it.

*DESCRIPTION:* 

Add a new package to github.com/vmware/govmomi library that covers the use cases and patterns identified as part of spike SPLAT-319 (doc).

Stakeholders for this Jira are owners of OpenShift projects that implement their own session management. The main stakeholders are identified in SPLAT-319.

*REQUIRED:*

• A PR that has been reviewed by stakeholders.
• A PR merged into github.com/vmware/govmomi containing the enhanced session management.
• If PR is denied by VMWare, submit to github.com/openshift/library-go (needs approval, will update)
• Unit tests

*ACCEPTANCE CRITERIA:* 

• Is new code accessible from OpenShift projects that previously used their own session management implementation?
• Is main PR merged?
• Are unit tests present and passing?
• Are new Jira tickets opened for OpenShift projects to consume new code?

*ENGINEERING DETAILS:*

If VMWare is not willing to accept code additions into github.com/vmware/govmomi then we should submit them into github.com/openshift/library-go; note that some value would be lost here since upstream k8s projects (vSphere CSI, vSphere CAPI) will likely not import OpenShift's library-go.