feat: introduce layered crate

[martintmk]

Summary

Introduces layered, a modern service abstraction crate for building composable async services with middleware.

Key features:

• Service trait using native async fn with &self - no manual Future types or boxing required
• Layer and Stack for composing middleware using ergonomic tuple syntax: (layer1, layer2, service).build()
• Execute helper to convert closures into services
• Intercept middleware for observing/modifying inputs and outputs (logging, metrics, validation)
• DynamicService for type erasure when concrete types become unwieldy
• Bidirectional Tower interoperability via the tower module

[github-actions]

⚠️ Breaking Changes Detected

error: failed to retrieve local crate data from git revision

Caused by:
    0: failed to retrieve manifest file from git revision source
    1: possibly due to errors: [
         failed when reading /home/runner/work/oxidizer/oxidizer/target/semver-checks/git-origin_main/bb696d98146cef7b01a76566973996c83f574451/scripts/crate-template/Cargo.toml: TOML parse error at line 9, column 26
         |
       9 | keywords = ["oxidizer", {{CRATE_KEYWORDS}}]
         |                          ^
       missing key for inline table element, expected key
       : TOML parse error at line 9, column 26
         |
       9 | keywords = ["oxidizer", {{CRATE_KEYWORDS}}]
         |                          ^
       missing key for inline table element, expected key
       ,
         failed to parse /home/runner/work/oxidizer/oxidizer/target/semver-checks/git-origin_main/bb696d98146cef7b01a76566973996c83f574451/Cargo.toml: no `package` table,
       ]
    2: package `layered` not found in /home/runner/work/oxidizer/oxidizer/target/semver-checks/git-origin_main/bb696d98146cef7b01a76566973996c83f574451

Stack backtrace:
   0: anyhow::error::<impl anyhow::Error>::msg
   1: cargo_semver_checks::rustdoc_gen::RustdocFromProjectRoot::get_crate_source
   2: cargo_semver_checks::rustdoc_gen::StatefulRustdocGenerator<cargo_semver_checks::rustdoc_gen::CoupledState>::prepare_generator
   3: cargo_semver_checks::Check::check_release::{{closure}}
   4: cargo_semver_checks::Check::check_release
   5: cargo_semver_checks::exit_on_error
   6: cargo_semver_checks::main
   7: std::sys::backtrace::__rust_begin_short_backtrace
   8: main

If the breaking changes are intentional then everything is fine - this message is merely informative.

Remember to apply a version number bump with the correct severity when publishing a version with breaking changes (1.x.x -> 2.x.x or 0.1.x -> 0.2.x).

[geeknoid]

For the logo, how about a crab thinking of a crab in a thought bubbken, and the crab in the thought bubble thinking of another crab in a thought bubble, etc.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (bae3cf9) to head (e0082bb).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #189    +/-   ##
========================================
  Coverage   100.0%   100.0%            
========================================
  Files          98      104     +6     
  Lines        6541     6784   +243     
========================================
+ Hits         6541     6784   +243     

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[ralfbiedert]

Sorry, I did not get to review this last week. That said, could we do a follow-up PR addressing a few issues:

• cargo doc gives me various warnings coming from layer, like warning: unresolved link to DynamicService --> crates/layered/src/lib.rs:113:40
• most tests should be moved to tests/
• the "Key Concepts" section is to short and doesn't quite make sense to me. For example, when you write "Service: An async function In → Out that processes inputs" you seem to want to say that a Service acts like a function (as it transforms some input to some output), but clearly it's a type with a trait implementation. I think all of these entries should be more clarified. I also wouldn't use the term "factory".
• The example below that ("Layers and Middleware") doesn't work out well what a middleware actually is. It seems to be left as an exercise for the reader
• Why is Execute called that? Shouldn't this be a ServiceFn or so?

Also, some docs look like so (apparently related to warnings above)

[geeknoid]

@ralfbiedert To make cargo doc happy, give it the --all-features options.