Add mock data for starting a transaction

[loan21xw]

As starting a transaction requires a charger to be connected, testing the starting and stopping of a charging session is hard. Instead, we can create a fake charger that always responds as if it was connected. This means that a specific chargerID can be used to test. What needs to be done:

• Respond with mock data on requests to POST /transactions, passing a specific transactionID every time, when a specific chargerID is used.
• Respond with mock data on requests to PUT /transactions/start/{transactionID} when the transactionID is a certain value.
• Respond with mock data on requests to GET /transactions/{transactionID} when the transactionID is a certain value.
• Respond with mock data on requests to PUT /transactions/stop/{transactionID} when the transactionID is a certain value.

Randomizing certain data in the responses might be a good idea.

[loan21xw]

Later it might be a good idea to create a "deeper" fake charger that behaves like a real charger, providing updates and such, but this is a good start and shouldn't take too long to implement.