Add code-only build support

[Jaytee-fn]

The change adds local build support for code-only functions.

This PR introduces a code-only build path that packages local function source into a zip archive instead of performing the traditional container image build flow.

The PR -

• detect 'code_only: true' in 'fn build'
• validate required local tooling before packaging
• create a local archive named:
  • ..zip
• bypass the legacy image/container-engine build path for code-only functions
• apply runtime-specific archive layout behaviour for code-only packaging

Validation

Build a code-only function
fn build --working-dir /path/to/code-only-function