api: add checksum validation in Content Library update session file API

[dilyar85]

Description

This PR updates the AddLibraryItemFileFromURI API with an optional LibraryItemFileChecksumInfo that the server can use to verify the uploaded library file content.
It also updates the govc library.import command accordingly with the new checksum arguments (-c for checksum value & -a for checksum algorithm).

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Updated the existing govc bats to verify the new arguments syntax. Passed all tests locally:
• Manually verified the new API change against a real VC:

# No checksum specified:
$ govc library.import -pull checksum-test https://releases.ubuntu.com/14.04.6/ubuntu-14.04.6-server-amd64.iso

$ govc library.ls checksum-test/
/checksum-test/ubuntu-14.04.6-server-amd64

# Invalid checksum algorithm:
$ govc library.import -a=foo -pull checksum-test https://releases.ubuntu.com/14.04.6/ubuntu-14.04.6-server-amd64.iso
govc: invalid checksum algorithm: foo

# Invalid checksum value:
$ govc library.import -c=invalid -pull checksum-test https://releases.ubuntu.com/14.04.6/ubuntu-14.04.6-server-amd64.iso
govc: The import of library item 2a57efd7-32be-4aad-80f6-4212f123a82c has failed. Reason: Error transferring file ubuntu-14.04.6-server-amd64.iso from https://releases.ubuntu.com/14.04.6/ubuntu-14.04.6-server-amd64.iso. Reason: Checksum mismatch for endpoint https://releases.ubuntu.com/14.04.6/ubuntu-14.04.6-server-amd64.iso in item ubuntu-14.04.6-server-amd64.iso: [actual checksum = b17d7c1e9d0321ad5810ba77b69aef43f0f29a5422b08120e6ee0576c4527c0e; expected checksum = invalid].

# Valid checksum value and algorithm:
$ govc library.import -c=b17d7c1e9d0321ad5810ba77b69aef43f0f29a5422b08120e6ee0576c4527c0e -a=SHA256 -pull checksum-test https://releases.ubuntu.com/14.04.6/ubuntu-14.04.6-server-amd64.iso

$ govc library.ls checksum-test/
/checksum-test/ubuntu-14.04.6-server-amd64

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 @dilyar85