Replace ReadContext.metadata with typemap.

[XAMPPRocky]

Currently the metadata field requires knowing the right string, and then knowing what type to downcast to. However we can actually encapsulate these things into the type system, and we can take it a step further and use types instead of strings to identify dynamic metadata, this is not only more efficient as it removes two levels of indirection in the lookup (Arc -> String -> str), it also guarantees that the value will always be the right type. Here's an example of how the capture bytes metadata would work.

pub struct CaptureBytes;

impl typemap::Key for CaptureBytes {
    type Value = Vec<u8>;
}

// in capture bytes using typemap::TypeMap
ctx.metadata.set::<CaptureBytes>(ctx.contents.clone());

// In another filter.
let capture_bytes: &Vec<u8> = ctx.metadata.get::<CaptureBytes>().unwrap();

[iffyio]

I like this idea, both for the perf and clarity/type-safety but I can imagine it'll be nice to have configurable keys and this won't be able to support that - I don't know of any use case that would require dynamic keys but I can't say at all how common it'll be either 🤷‍♂️ I'm thinking maybe we can start with this and if it turns out that some use case requires dynamic string we can solve that as we go (maybe use a separate map or go back to the current solution or something else entirely)? But as of today I'll be up for changing to use typemap

[XAMPPRocky]

Well my current hesitation is over CaptureBytes's utility with this change. Currently with capture byte's configurable keys you can capture data into different buckets, where as with type map you wouldn't be able to do that. so maybe we need take another look at how we're handling dynamic metadata a bit closer, because there are going to be other filters that want to capture, so maybe we can build that assumption into how you set metadata.