feat: allow overriding the browserlayer

[erral]

For full compatibility with cookieplone, we need to be able to override the BrowserLayer, we can pass it in the bobtemplate.cfg file like the `package.dottedname.

See https://github.com/plone/cookieplone-templates/pull/248 for changes in cookieplone

[davisagli]

There's a failing test. It appears to be already failing on main...