Add support of SolR basic authentication

[tisto]

@mpeeters can you elaborate on the use case for basic auth? Solr usually runs behind a firewall and is only accessible from the server itself in the first place. I am not against merging this PR, I am just wondering about the reasoning.

[mpeeters]

@tisto the use case is pretty the same has having authentication for PostgreSQL when using relstorage, this allow to avoid having a Plone site accessing other core when using a SolR cluster hosting multiple cores.

BTW, I will do an another PR to allow search accross multiple cores in next few weeks.

[tisto]

@mpeeters would it be possible to rename the username/password field to solr_username/solr_password? Maybe I am the only person with that problem but my password manager and browser always fill out those fields automatically and that's pretty annoying. We could avoid this by just renaming the fields I guess. What do you think?

[mpeeters]

Hello @tisto, I see your point and I will make a new PR with those changes