Update for week 1 of Milestone 3

Last week I managed to do the following tasks for #SAOC2020:
- removed unused imports
- added user defined attributes for reqbody classes
- improved bodyToJSON function:
    1. instead of converting all the fields, the function converts only those that are
       allowed for the operation executed. The fields that are used for an operation have
       its name as an attribute. For example, if I want to create a file, the function
       will convert only those fields that have the "create" attribute.
    2. created a filter that removes methods names from the members names list
    3. conversion of array of objects and SysTime
- replaced bool from reqbody with Nullable!bool, because I want to use the fields that are initialized by the user

The plan for the next week:
- code refactoring
- add unittests
- test the entire library

Milestone 1 updates: [0]
Milestone 2 week 1: [1]
Milestone 2 week 2: [2]
Milestone 2 week 3: [3]
Milestone 2 week 4: [4]
Code repo: [5]

[0]: https://forum.dlang.org/post/eaefvlbgikujonfjjzqi@forum.dlang.org
[1]: https://forum.dlang.org/post/jezqwzpeknpxhmqgkewe@forum.dlang.org
[2]: https://forum.dlang.org/post/kffvsmmxrkftxlbqklsd@forum.dlang.org
[3]: https://forum.dlang.org/post/yvbnsycuxqjfxghqajbu@forum.dlang.org
[4]: https://forum.dlang.org/post/jwasbvtjbofftybnsqro@forum.dlang.org
[5]: https://github.com/Robert-Aron293/SAoC---Google-APIs/tree/master/dlang-google-api-interact