Customizable user provided call filters

[arrudagates]

We need an efficient system for fully customizable user provided call filters for each track, similar to the current permissions system, but with an extensive rule system.
In the past, a system was implemented with WASM functions, but that seems rather wasteful for this use case. Similarly, JSONLogic was explored (this is used as the frontend rendering logic for RMRK NFTs), but while this seems like a decent compromise, I couldn't find a maintained Rust implementation supporting no-std, so we would either need to fork and add no-std support ourselves or look for something else.
This discussion will be used to track ideas and possible implementations for this.

[arrudagates]

Rhai is a good contender here, offers decent performance, is well maintained, allows for simple syntax, compiles to AST for efficient storage, offers a decent level of type enforcement and compiles to no-std/wasm.

[arrudagates]

The problem with this approach is that we're sacrificing a lot of performance compared to WASM, because a bytecode engine will always be faster than an AST walker. Which comes back to the original reason not to use WASM and suddenly makes WASM seem crazy efficient here...

[arrudagates]

I believe WASM is worth exploring once again.

[arrudagates]

Wasmi should be explored again, this time it would be interesting to try to provide a higher level interface so that the users can write WASM to interact with the decoded data instead of the raw encoded call data.

[arrudagates]

One interesting idea here is to use pallet-contracts itself, since it already provides a very robust implementation of user provided WASM code. One thing missing in pallet-contracts (which we can contribute upstream if we implement it) is the ability to define contract filters in the runtime, so that our implemented pallet-contracts can only accept WASM code that follow specific conventions, like no using memory and having a main function that can only return a boolean.