Provide site capabilities to build plugins

[ascorbic]

Which problem is this feature request solving?

Currently a build plugin does not know whether a site can use gated features such as background functions. If this were enabled then we could catch this at build time and give a helpful message, rather than waiting until it fails during the deploy.

Describe the solution you'd like

Provide the site capabilities to plugins as part of the netlifyConfig object.

Pull request (optional)

• I can submit a pull request.

[github-actions]

This issue has been automatically marked as stale because it has not had activity in 1 year. It will be closed in 14 days if no further activity occurs. Thanks!