api: Expose vSphere API to evict subscribed content library

[acharyasreej]

Closes: #3400

Description

This change exposes the vSphere API to evict the subscribed content library to free up storage capacity.
The vSphere API evicts the cached content of an on-demand subscribed library. This operation allows the cached content of a subscribed library to be removed to free up storage capacity. This operation will only work when a subscribed library is synchronized on-demand.

Closes: #3400

Type of change

Please mark options that are relevant:

• Bug fix (non-breaking change which fixes an issue)
• 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?

Ran the govc tests locally

acharyasreej@acharyasreY5WYK test % ./library.bats
library.bats
✓ library
✓ library.import
✓ library.deploy
✓ library.clone ovf
✓ library.deploy vmtx
✓ library.vmtx.info
✓ library.pubsub
✓ library.subscriber example
✓ library.create.withpolicy
✓ library.findbyid
✓ library.trust
✓ library.session
✓ library.probe
✓ library.evict

Checklist:

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

[dougm]

lgtm, thanks @acharyasreej !