Add convert_to_base_64 function to convert string integer to magnitudes

[sahmad98]

Added a function to convert BigInt string representation into Base64 magnitudes. It starts from back of string and checks for overflow if saved into a 64-bit variable, which means we have reached at a point in string which is a component in magnitude vector, push this component into the vector and then start over again from the current point in string to check for next component.