Allow options in driver for escaping and unescaping string keys containing dollar and dot with preferred substitution

Currently we have to do manual work arounds when storing dictionaries of type Dictionary<string,?> in Mongo as there will be runtime exceptions if the string key ever contains a dollar or dot. We would like the option to specify an escape sequence for these characters in the driver. E.g. We currently use the \x<ascii-hexcode> format for escaping them ($ = \x24 . = \x2E)