api: Add library item storage APIs

[dilyar85]

Description

This PR adds a new set of APIs to retrieve library item storage, as defined in https://developer.broadcom.com/xapis/vsphere-automation-api/v8.0U1/content/library/item-storage/

Closes: N/A

Type of change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [ ] Build related change

How Has This Been Tested?

Consumed this API in an operator running in the internal Supervisor cluster and verified the expected Storage URI retrieved for each file available in a content library item:

fileInfo:
  - cached: true
    name: ubuntu-20.04-vmservice-v1alpha1.20210528.ovf
    sizeInBytes: "10552"
    storageURI: ds:///vmfs/volumes/... <redacted>
    version: "1"
  - cached: true
    name: ubuntu-20.04-vmservice-v1alpha1.20210528-disk1.vmdk
    sizeInBytes: "1426063360"
    storageURI: ds:///vmfs/volumes/... <redacted>
    version: "1"

Checklist:

• [x] My code follows the CONTRIBUTION guidelines of this project
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] (N/A) I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged