Improvements in the current implementation

Following are the necessary improvements in the ZorgTTP application:

• C1 Handling of data should be changed from string manipulation to JSON (src/main.rs:118-121, 176, 178)
• C2 Store data to a database (i.e., sqllite) instead of file (src/main.rs:33, 133)
• C3 Change IRMA client taken from https://github.com/tweedegolf/irmars to (hyper) client (src/main.rs:85, 143)
• C4 Structure endpoint response data in struct.

Based on these changes, chained session can be added in the application. C3 and C4 is especially necessary for chained session as there is no support for extended session request in the existing IRMA client.