[Rendering] Custom renderer support

[anuj-pal27]

Summary

Implements the config.renderer(:name) { ... } DSL proposed in the issue.
Registers custom renderer blocks and generates render_<name> methods on
RageController::API at boot time.

Key behavior

• config.renderer(:csv) { |obj, **opts| ... } registers a renderer block
• After __finalize, every controller automatically gets render_csv, render_phlex, etc.
• Generated methods support status: as both Symbol (:created) and Integer (201)
• Unknown status symbols raise ArgumentError instead of setting nil status
• Renderer blocks run in controller context — headers, request, params all work
• Double-render is prevented whether render happens via return value or internal render call
• RageController::API.__custom_renderers registry makes finalize idempotent across re-runs

Tests

9 specs covering:

• Basic registration and method generation
• Status handling (symbol, integer, default 200)
• Duplicate renderer registration guard
• Method name conflict detection
• Controller context access (headers, params)
• nil return value → empty string body
• Renderer block calling render internally
• Double-render protection

Notes

Renderer blocks are converted to named methods via define_dynamic_method rather
than using instance_exec per request — this keeps the hot path allocation-free
and consistent with how Rage handles action methods internally.

Fixes: #234

Screenshots

CSV renderer:

Phlex renderer: