Allow bidirectional exchange of files

[nidhal-labidi]

Checklist

• Resolves Allow bidirectional exchange of files #110
• Labelled and assigned a reviewer

Reviewer

• I've checked out the code and tested it myself.

Changes

• Refactor the code of the classes: FlottformFileInputHost and FlottformFileInputClient in a way that allows multiple use cases to be built on top of it.
• Allow bidirectional exchange of files.

[nidhal-labidi]

Will be divided into 2 separate PRs:

• feat/bidirectional-file-exchange --> feat/flottform-bidirectional-data-transfer: Contains changes for the FlottformFileInput classes only. It provides the changes necessary to send multiple files in both directions.

• feat/bidirectional-file-exchange-example --> feat/bidirectional-file-exchange: Contains an example of how the bidirectional file transfer can work in practice.

[nidhal-labidi]

Please refer to the previous comment and the following PRs to see how Flottform implemented the new API and dealt specifically with bidirectional file transfer:

1. Update FlottformFileInput classes to allow bidirectional file transfer #123
2. Bidirectional file exchange example #124
3. Update File exchange demos to use the new UI #125