feature: extend tauri builder with updater_header method

[Atrox]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

This change extends tauri::Builder with a new method updater_header (in the same vein as the existing updater_target) to set headers on the default updater instance.

[amrbashir]

Thanks for your contribution, but since updater in v2 has been moved to its own plugin, and since we already provide a similar API https://docs.rs/tauri/latest/tauri/updater/struct.UpdateBuilder.html#method.header, I will close this PR in order to avoid another breaking change.