Ability to provide alternative implementation of host functions

[xlc]

required by AcalaNetwork/chopsticks#13

I will start with some refactoring on host.rs, abstract the host handling code into a trait, move the current implementation to a struct, and then make it customizable.

[tomaka]

abstract the host handling code into a trait

There are two traits in the entire source code right now, and it's because I had no other choice. If you open a PR that adds trait definitions, I'll likely refuse it. I don't want smoldot to turn into Substrate with its hundreds of incomprehensible abstractions.

If you explain what you would like to achieve, I can provide a code design for it.
If you just want to disable signatures verification, a simple force_signatures_verifications_success: bool field in host::Config suffices.

[xlc]

Ideally I want user to be able to override host functions but I am ok to have this less generic at this stage.

So I will add a config mock_signature_verification_host_functions, when true, it will return true if the signature is valid OR if the signature is equal to a magic value. This will be safe to leave enabled for a test client that it will still reject most of the invalid signatures.

[xlc]

closed by #2955