fix: improve nested configuration field support

This fixes the parser introduced in https://github.com/falcosecurity/falco/pull/1792.
Now, nested fields such as `arr[1].subval` are supported, whereas the parser used
to recognize the `.` as an unexpected character.

Signed-off-by: Jason Dellaluce <jasondellaluce@gmail.com>

userspace/falco/configuration.h

@@ -168,6 +168,10 @@ private:
 					int nodeIdx = std::stoi(key.substr(i + 1, close_param_idx - i - 1));
 					ret.reset(ret[nodeIdx]);
 					i = close_param_idx;
+					if (i < key.size() - 1 && key[i + 1] == '.')
+					{
+						i++;
+					}
 				}
 			}
 		}