Allow pluggable Authentication Providers in BasicAuthPlugin

Currently Sha256AuthenticationProvider is hardcoded in BasicAuthPlugin. Allow user to specify a new parameter authenticationProvider in security.json which determines the authentication provider with . Example:

"authentication":{
   "blockUnknown": true,
   "class":"solr.BasicAuthPlugin",
   "provider":{
        "class":"my.custom.AuthenticationProvider",
        "param":"value"
    }
}

We would still default to Sha256AuthenticationProvider