Add JSON Schema generation for ECS application config

[madmecodes]

Summary

• Adds a tool/jsonschema-gen Go program that generates JSON Schema from PipeCD configuration structs using github.com/invopop/jsonschema
• Generates schema for ECSApplicationSpec with camelCase field names matching the YAML config format
• Output goes to docs/static/jsonschema/ecs.json, ready to be served at a public URL for editor integration

Closes #6684

How it works

go run ./tool/jsonschema-gen -out docs/static/jsonschema

Users can then reference the schema in their YAML configs for autocompletion and validation:

# yaml-language-server: $schema=https://pipecd.dev/jsonschema/ecs.json

Test plan

• Tool compiles and runs successfully
• Generated schema includes all ECS-specific types (ECSDeploymentInput, ECSTargetGroups, ECSSyncStageOptions, etc.)
• Field names use camelCase matching the json struct tags
• Verify schema works with Red Hat YAML Language Server in VS Code