auth: expose backend storage layout version when relevant

[miodvallat]

Short description

This PR adds some plumbing for backends to be able to report a "storage version number", whatever that means on a per-backend basis. This data, when available, is made available through the HTTP API via a currently undocumented endpoint.

The LMDB backend implements this to return the schema version currently used for the given zone.

The lack of documentation is intentional, for I am not sure this is something useful to users at this point.

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document, including the AI Policy, and added a "Signed-off-by" to my commits
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Pull Request Test Coverage Report for Build 19433481725

Details

• 28 of 28 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall first build on piglatin at 73.045%

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Totals
Change from base Build 19426528573:	73.0%
Covered Lines:	127995
Relevant Lines:	164499

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💛 - Coveralls