Allow disabling of extraspath with solrconfig.xml if not allowed by environment

[chillu]

For security reasons, we don't allow customisations to solrconfig.xml in CWP (due to the shared service nature of the Solr service). This often throws off devs who just read the open source module docs, and assume they have full configuration power over the lowlevel Solr details (as described in configuration docs). While we can counter this through cwp.govt.nz developer docs, ideally we can fail hard if devs try to customise this stuff to avoid them spending time on advanced features which then can't be deployed onto CWP.

I'm raising this here on the module since I assume it'll require module changes to allow for this opt-in warning.

Internal reference: https://silverstripe.atlassian.net/browse/CWP-1119

/cc @mateusz