feat: add new GetAllMaintainedLinuxSIGImageConfigTemplates API

[cameronmeissner]

What type of PR is this?

What this PR does / why we need it:

details from new API description:

// GetAllMaintainedLinuxSIGImageConfigTemplates returns the SIG image config templates associated
// with all currently-maintained Linux images. In this case, "currently-maintained" denotes that the
// image is currently built and released during official SIG releases. This is 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