feat: add GB300 VHD SKU to AgentBaker. Disable building it by default until we need it.

[abenn135]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Adds GB300 VHD SKU. Disabled by default.

Which issue(s) this PR fixes:

Fixes #

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version
• commits are GPG signed and Github marks them as verified

Special notes for your reviewer:

Release note:

none