feat: bundle field definitions from required input packages

[teresaromero]

Bundle field definitions from required input packages

Closes elastic#3280

Composable packages stack — part 3 of 4.
This PR is stacked on elastic#3329 (bundle templates, closes elastic#3278) and #1 (merge variables, closes elastic#3279). Merge order: elastic#3329 → #1 → this PR → #3.
Each PR will be rebased onto main once its predecessor merges.

Summary

Extends the required-inputs build step to bundle field definitions from required input packages into the composable integration's data streams.

Bundling rules:

• Fields already defined in the integration take precedence; only absent fields are copied from the input package.
• Bundled fields are written to a new file <inputPkgName>-fields.yml inside the data stream's fields/ directory, keeping integration-owned field files untouched.

What changed

internal/requiredinputs/fields.go (new, ~200 lines)

• BundleFields: entry point called as part of the required-inputs build step.
• Reads field definitions from the input package, compares against integration-defined fields, and writes missing ones.

internal/requiredinputs/fields_test.go (new, ~300 lines)

• Covers: full field bundle, partial override (integration fields win), no-op when all fields present.

test/manual_packages/required_inputs/ (new fixtures)

• fields_input_pkg — input package with a set of base field definitions.
• with_field_bundling — integration that declares a subset of fields, expecting the rest to be bundled in.

How to test

cd test/manual_packages/required_inputs/fields_input_pkg
elastic-package build

elastic-package stack up --version 9.4.0-SNAPSHOT

cd test/manual_packages/required_inputs/with_field_bundling
elastic-package build
# Inspect data_stream/*/fields/ — should contain <inputPkgName>-fields.yml with bundled fields

Dependencies

• bundle templates of required input packages elastic/elastic-package#3329 and feat: merge variables from required input packages into composable manifests #1 must merge first.