dnsdist: Unify certificate reloading syntaxes

[rgacogne]

Short description

This PR unifies the way of reloading certificates over the different types of frontends (DNSCrypt, DoT and DoH). It also paves the way for the deprecation of the TLSCtx class by making every method available from the TLSFrontend class.

Checklist

I have:

• read the CONTRIBUTING.md document
• 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)