Add experimental Overlaybd API server pre-auth

[pakkah]

What this PR does / why we need it:

This PR adds an experimental Overlaybd API server pre-auth hook in the
snapshotter path.

When the experimental pre-auth feature is enabled, the snapshotter
notifies the Overlaybd API server with the generated overlaybd config
before attaching the device. This allows the API server to warm up auth
state ahead of the actual attach path, which helps reduce auth-related
latency during device setup.

The PR also adds the related experimental configuration fields and
default values, updates the documentation, and includes unit tests for
the new config behavior.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Please check the following list:

• Does the affected code have corresponding tests, e.g. unit test, E2E test?
• Does this change require a documentation update?
• Does this introduce breaking changes that would require an announcement or bumping the major version?
• Do all new files have an appropriate license header?