Allow async onCreatePublication callbacks

[smoores-dev]

Follow up to the discussion here.

Allows onCreatePublication to be async, so that consumers can utilize async methods (e.g. reading a resource's contents) in the callback.

[Copilot]

Pull request overview

This PR enables asynchronous operations within onCreatePublication callbacks by converting the Publication.Builder.Transform typealias and related methods to async. This allows consumers to use async methods (e.g., reading resource contents) when modifying publication components during construction.

Key changes:

• Made Publication.Builder.Transform typealias async
• Updated Publication.Builder.apply() method to be async and await the transform
• Updated PublicationOpener to await when applying transforms

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
Sources/Shared/Publication/Publication.swift	Modified Transform typealias and apply() method to support async callbacks
Sources/Streamer/PublicationOpener.swift	Updated transform application to use await with the now-async apply() method
CHANGELOG.md	Documented the async callback support feature

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[mickael-menu]

Looking good, thank you @smoores-dev!