dnsdist: Add Lua bindings to be able to rotate DNSCrypt keys

[rgacogne]

Short description

This makes it possible to rotate DNSCrypt certificates from the console, either manually or via some sort of scheduled job.
It is also possible to use the maintenance() function to generate in-memory certificates and rotate them automatically from Lua. Such an example can be found in the regression tests.
Closes #5420.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled and tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[publicarray]

Thanks so much!