Support starting transaction in EvseManager

[golovasteek]

resolves #572

The solution presented here is suitable for basic use-case under German Measurement law.

Prior to starting charging on BSP, the EvseManager will request meter to start transaction.
Starting the transaction on the power meter is expected to return quicly (by either spawning some process in the meter driver, or initiating the transaction on the meter hardware).

The signed meter values for start transaction are not expected to be reported by the meter when transaction is requested.

There are different meter models, and signed meter values for transaction start, may be a separate "SignedMeterValue" object. in this case the start_signed_meter_value of "StopTransactionResponse" can be used. And the signed_meter_value would contain data for the session finish.

In many cases the meter only issue one signed dataset for session, that contains multiple snapshots encoded, in this case only signed_meter_value is used.

What is not covered by this PR:

• OCPP module as well as libOCPP need to be adjusted, to use "start_signed_meter_value" from TransactionFinished event
• the meter values for the transaction are only transferred at the end of the session, there is no support for asynchronous sending of meter values introduced. Which needs to be designed spearately, once we have better requiremetns from the OCPP of different versions.

[corneliusclaussen]

Next time if you change something during review, please do not squash commits. It is very hard to follow your changes and see if they are resolved now. The best way is to squash before opening the PR, then stack commits with changes during review on it and finally squash again as needed during the merging.

[golovasteek]

Next time if you change something during review, please do not squash commits. It is very hard to follow your changes and see if they are resolved now. The best way is to squash before opening the PR, then stack commits with changes during review on it and finally squash again as needed during the merging.

Makes sense.
Will do this way.