Refactor Consumer Module: Introduce JobRunner Trait for Flexible Job Handling

[Kab1r]

Background

I need to have access to some initialized configuration / state for my Handler.
While I could use once_cell to initialize static variables to hold a configuration, it isn't particularly idiomatic.

Summary of Changes

This PR introduces a change to the Consumer module, converting the JobRunner type into a trait.
The JobRunner trait is now implemented for types that can handle jobs, providing more flexibility for handling jobs within a Consumer.

Example Usage

use faktory::{ConsumerBuilder, JobRunner, Job};
use std::io;

struct MyHandler {
    config: String,
}
impl JobRunner<io::Error> for MyHandler {
   fn run(&self, job: Job) -> Result<(), io::Error> {
      println!("config: {}", self.config);
      println!("job: {:?}", job);
      Ok(())
  }
}

let mut c = ConsumerBuilder::default();
let handler = MyHandler {
   config: "foo".to_string(),
};
c.register_runner("bar", handler);
let mut c = c.connect(None).unwrap();
if let Err(e) = c.run(&["default"]) {
    println!("worker failed: {}", e);
}

---

This change is 

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (cdf15d3) 68.4% compared to head (6ed657f) 68.2%.

Additional details and impacted files

Files	Coverage Δ
src/consumer/mod.rs	68.4% <57.1%> (-1.3%)	⬇️

[jonhoo]

I totally see the need! Left some comments on how to achieve it though.

[jonhoo]

Almost there — just a couple of nits left!

[jonhoo]

Just one more bit to avoid the box, and then I think we're done!

[jonhoo]

Looks good to me — thanks for persisting!

[jonhoo]

Released in 0.12.5 🎉