Add netlifyConfig.headers

[ehmicky]

Plugin authors should be able to access the site's headers (set either in netlify.toml or in _redirects) using the netlifyConfig.headers argument.

This is a requirement for #1193, since we cannot make netlifyConfig.headers mutable if that property does not exist.

Additionally, this property should be mutable.

Most of the logic in @netlify/config and @netlify/build (including detecting whether the config file was added by the build command, or whether to remove the _headers file at the end of the build) can be copied from redirects.

[ehmicky]

PR at #3407.

[ehmicky]

Done!