make x-initialSync part of the spec as initial_sync attribute

[glours]

x-initialSync attribute was there for more than a year as an experimental feature to synchronize host content with services containers when sync+x triggers are setup, we think it could be now officially part of the specification

Compose Specification PR

See docker/docs#23345

[Copilot]

Pull Request Overview

This PR formalizes the experimental x-initialSync attribute by promoting it to an official initial_sync attribute in the Compose specification. This attribute ensures initial synchronization is performed before starting watch mode for sync+x triggers.

• Adds initial_sync as a boolean field to the Trigger struct
• Updates the JSON schema to include the new initial_sync property with proper documentation
• Migrates existing test cases from the experimental x-initialSync to the new initial_sync attribute

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
types/develop.go	Adds InitialSync boolean field to the Trigger struct with YAML/JSON tags
types/derived.gen.go	Updates the generated deep copy function to include the new InitialSync field
schema/compose-spec.json	Adds initial_sync boolean property to the JSON schema with description
loader/loader_test.go	Updates test case to use the new initial_sync attribute instead of x-initialSync

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