Adding unit tests for DIN70121 messages

### ISO Version

DIN70121

### Describe the problem

It would be very useful for the library to have unit tests for each V2G message.

In the following table you will find the XML file and the corresponding EXI stream for each DIN message.

| Message | Files | Done |
| -------- | ------- | ------- |
| SessionSetupReq | [SessionSetupReq.zip](https://github.com/EVerest/libcbv2g/files/14536321/SessionSetupReq.zip) | :heavy_check_mark: |
| SessionSetupRes | | :x: |
| ServiceDiscoveryReq | [ServiceDiscoveryReq.zip](https://github.com/EVerest/libcbv2g/files/14536511/ServiceDiscoveryReq.zip) | :heavy_check_mark: |
| ServiceDiscoveryRes | [ServiceDiscoveryRes.zip](https://github.com/EVerest/libcbv2g/files/14536522/ServiceDiscoveryRes.zip) | :heavy_check_mark: |
| ServicePaymentSelectionReq | [ServicePaymentSelectionReq.zip](https://github.com/EVerest/libcbv2g/files/14537075/ServicePaymentSelectionReq.zip) | :x: |
| ServicePaymentSelectionRes | [ServicePaymentSelectionRes.zip](https://github.com/EVerest/libcbv2g/files/14537076/ServicePaymentSelectionRes.zip)| :x: |
| ContractAuthenticationReq | [ContractAuthenticationReq.zip](https://github.com/EVerest/libcbv2g/files/14537081/ContractAuthenticationReq.zip) | :x: |
| ContractAuthenticationRes | [ContractAuthenticationRes.zip](https://github.com/EVerest/libcbv2g/files/14537083/ContractAuthenticationRes.zip)| :x: |
| ChargeParameterDiscoveryReq | [ChargeParameterDiscoveryReq.zip](https://github.com/EVerest/libcbv2g/files/14537087/ChargeParameterDiscoveryReq.zip)| :x: |
| ChargeParameterDiscoveryRes | [ChargeParameterDiscoveryRes.zip](https://github.com/EVerest/libcbv2g/files/14537090/ChargeParameterDiscoveryRes.zip) | :x: |
| CableCheckReq | [CableCheckReq.zip](https://github.com/EVerest/libcbv2g/files/14537097/CableCheckReq.zip) | :x: |
| CableCheckRes | [CableCheckRes.zip](https://github.com/EVerest/libcbv2g/files/14537103/CableCheckRes.zip)| :x:|
| PreChargeReq | [PreChargeReq.zip](https://github.com/EVerest/libcbv2g/files/14537127/PreChargeReq.zip) | :x: |
| PreChargeRes | [PreChargeRes.zip](https://github.com/EVerest/libcbv2g/files/14537130/PreChargeRes.zip)| :x: |
| PowerDeliveryReq | [PowerDeliveryReq.zip](https://github.com/EVerest/libcbv2g/files/14537134/PowerDeliveryReq.zip) | :x: |
| PowerDeliveryRes | [PowerDeliveryRes.zip](https://github.com/EVerest/libcbv2g/files/14537140/PowerDeliveryRes.zip) | :x: |
| CurrentDemandReq | [CurrentDemandReq.zip](https://github.com/EVerest/libcbv2g/files/14537158/CurrentDemandReq.zip)| :x: |
| CurrentDemandRes | [CurrentDemandRes.zip](https://github.com/EVerest/libcbv2g/files/14537159/CurrentDemandRes.zip) | :x: |
| WeldingDetectionReq | [WeldingDetectionReq.zip](https://github.com/EVerest/libcbv2g/files/14537161/WeldingDetectionReq.zip) | :x: |
| WeldingDetectionRes | [WeldingDetectionRes.zip](https://github.com/EVerest/libcbv2g/files/14537162/WeldingDetectionRes.zip) | :x: |
| SessionStopReq | [SessionStopReq.zip](https://github.com/EVerest/libcbv2g/files/14537166/SessionStopReq.zip)| :x: |
| SessionStopRes | [SessionStopRes.zip](https://github.com/EVerest/libcbv2g/files/14537170/SessionStopRes.zip)| :x: |

### Describe your solution

Create for every message a encode and decode test with the help of XML and EXI messages.

A sample of how the tests can be written can be found in the `tests/app_handshake` folder.

### Additional context