feat: add dedicated pipeline variable to determine whether to publish VHD blobs

[cameronmeissner]

What type of PR is this?

What this PR does / why we need it:

this PR does the following:

• add a new pipeline variable GENERATE_PUBLISHING_INFO with different default values set on respective pipelines (default value for check-in pipeline + test-all pipeline is now false, official build pipeline default is true)
• with this change, we no longer publish VHD blobs to a storage account unless running an official release, which is the only time we'd need to actually publish any blobs (packer will always publish to SIG, so we will maintain the ability to build VHDs for agentbaker e2e's in all pipelines)
• DRY_RUN is preserved now as a pipeline variable, also with different default values based on the pipeline, this variable now only dictates whether to preserve the SIG image version packer generates as the build output
• some other cleanup/renaming

TLDR - this change should drastically reduce storage costs by only having VHD blobs be published as a part of official builds, or if a dev explicitly overrides GENERATE_PUBLISHING_INFO via pipeline variables

Which issue(s) this PR fixes:

Fixes #

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Special notes for your reviewer:

Release note:

none