Add channel_config hash to allow extension-level fields

[Fernanda-Montiel]

WHY are these changes introduced?

To support extension-level configuration related to channels lifecycle. The extension is under a flag, so this is a non-breaking change

WHAT is this pull request doing?

Adds an optional channel_config field to the ChannelSpecificationSchema and includes it in the returned configuration object. This allows channel-specific configuration to be stored in extension specifications.

How to test your changes?

1. Create a channel extension specification with a channel_config property
2. Verify that the configuration is properly parsed and returned
3. Ensure existing channel specifications without this field continue to work

Measuring impact

How do we know this change was effective? Please choose one:

• [x] n/a - this doesn't need measurement, e.g. a linting rule or a bug-fix
• [ ] Existing analytics will cater for this addition
• [ ] PR includes analytics changes to measure impact

Checklist

• [x] I've considered possible cross-platform impacts (Mac, Linux, Windows)
• [x] I've considered possible documentation changes

[Fernanda-Montiel]

• #6620 👈 (View in Graphite)
• main

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[github-actions[bot]]

We detected some changes at packages/*/src and there are no updates in the .changeset. If the changes are user-facing, run pnpm changeset add to track your changes and include them in the next release CHANGELOG.

[!CAUTION] DO NOT create changesets for features which you do not wish to be included in the public changelog of the next CLI release.

[Fernanda-Montiel]

Closing in favour of https://app.graphite.com/github/pr/Shopify/cli/6628/Allow-to-create-contract-based-specifications-with-some-local-overrides?ref=gt-pasteable-stack