feat: Builder-style API for App

[spencewenski]

Builder-style APIs are common in rust libs. For example, see axum's Router or poem's Route. This is often a more ergonomic approach to building an object as it allows creating state in local variables and passing them to the builder methods.

To add a builder-style API will still supporting our App trait, we provide a default App implementation (RoadsterApp) that can be configured via a builder-style API. This allows consumers to choose which approach they prefer.

Closes #339

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 226 lines in your changes missing coverage. Please review.

Project coverage is 43.80%. Comparing base (819bad3) to head (c4f4ed8).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/app/roadster_app.rs	0.00%	221 Missing ⚠️
src/app/mod.rs	0.00%	4 Missing ⚠️
src/service/http/builder.rs	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/service/runner.rs	0.00% <ø> (ø)
src/service/http/builder.rs	26.92% <0.00%> (+0.50%)	⬆️
src/app/mod.rs	0.00% <0.00%> (ø)
src/app/roadster_app.rs	0.00% <0.00%> (ø)

... and 3 files with indirect coverage changes

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