Allow closures in #config_target's to be serialised

Pushed an MR that implements the functionality. We need to decide how to set the secret.

Probably it could help to serialize better renderables too, lots of callbacks in core pointing to services or static methods (not really callables) and can't use closures

Probably it could help to serialize better renderables too, lots of callbacks in core pointing to services or static methods (not really callables) and can't use closures

I agree, it can probably help there as well, but we can leave that to a followup.

 // Configure Serializable Closure signing for security.
    // @todo Use hash salt and private key - but it is not always available
    //   here.

This is the comment in the code about setting the secret, I don't understand why the hash salt is not available there.
Can we use it when it exists and fall back to not serializing when it is not available?

@borisson_ example of when we don't have a hash salt:

• in the very early installer
• in unit tests.

Also I wonder if we should be using the private key here too which would mean a database query.

Revisiting this, reading the comments seems like using private key is our only option right?