Add radius to lexer/parser

src/bin/dhcp4/dhcp4_lexer.ll

@@ -311,6 +311,15 @@ ControlCharacterFill            [^"\\]|\\{JSONEscapeSequence}
     }
 }
 
+\"radius\" {
+    switch(driver.ctx_) {
+    case isc::dhcp::Parser4Context::DATABASE_TYPE:
+        return isc::dhcp::Dhcp4Parser::make_RADIUS(driver.loc_);
+    default:
+        return isc::dhcp::Dhcp4Parser::make_STRING("radius", driver.loc_);
+    }
+}
+
 \"user\" {
     switch(driver.ctx_) {
     case isc::dhcp::Parser4Context::LEASE_DATABASE:

src/bin/dhcp4/dhcp4_parser.yy

@@ -70,6 +70,7 @@ using namespace std;
   MYSQL "mysql"
   POSTGRESQL "postgresql"
   CQL "cql"
+  RADIUS "radius"
   USER "user"
   PASSWORD "password"
   HOST "host"