feat: add new GetMaintainedLinuxSIGImageConfigMap API

[cameronmeissner]

What type of PR is this?

What this PR does / why we need it:

details from new API description:

// GetMaintainedLinuxSIGImageConfigMap returns a set of Distro -> SigImageConfig mappings
// for ALL Linux distros that are currently built and maintained by AKS Node SIG (Version == LinuxSIGImageVersion).
// Note that each distro's SigImageConfig SubscriptionID field will be empty.
// This can be used downstream to make sure that all expected images have been properly replicated before
// releasing new versions of agentbakersvc.

will be used by downstream code to safely release agentbakersvc

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

[Devinwong]

lgtm