Add code-only function init and runtime discovery support

[Jaytee-fn]

This PR introduces the first slice of code-only function support in Fn CLI.

It adds:

1. code-only 'fn init'
2. func.yaml/schema support for code-only fields
3. runtime discovery commands inside Fn CLI

This allows users to initialize code-only functions using the new schema/version and discover valid managed runtimes and runtime versions without leaving Fn CLI.

Included in this PR

Code-only init

1. support for 'fn init --code-only'
2. support for: --runtime-name and --runtime-config-type
3. code-only func.yaml generation using: schema_version: 20260325
4. generation of runtime-specific default handler values for code-only functions

Funcfile/schema support

1. support for code-only func.yaml fields in the internal funcfile model
2. schema support for: code_only, runtime_config and handler

Runtime discovery

1. fn list runtimes
2. fn list runtime-versions --runtime-name
3. fn get latest-runtime-version --runtime-name

### Runtime-specific code-only init behaviour
For code-only init, runtime handling is explicit.

Python / Node

Python and Node use custom code-only boilerplate generators because the existing helpers produce file names and conventions tied to older image-based flows.

Java / Go

Java and Go also go through explicit code-only init branches, but delegate to the existing shared language helpers

This keeps the code-only init flow more uniform across supported runtimes while still reusing the existing language generators where appropriate.

### Validation

Code-only init
fn init --code-only --runtime-name python311.ol9 --runtime-config-type function-update
cat func.yaml

schema_version: 20260325
code_only: true
runtime_config:
type: function-update
runtime_name: python311.ol9
handler: hello_world.handler

Runtime discovery
fn list runtimes
fn list runtime-versions --runtime-name python311.ol9
fn get latest-runtime-version --runtime-name python311.ol9

This PR establishes the initialization and runtime discovery for code-only workflows. The PR intentionally leaves the existing legacy versioned internal type names (for example 'FuncFileV20180708' and 'RuntimeConfigV20180708') unchanged. Renaming these to version-agnostic names is planned as a follow-up cleanup PR.