Add support for custom JSON encoders for serialising of the session

[jesper-friis]

Even though the issue is in oteapi-services, maybe the correct place to add an option for the user to provide a custom JSON encoder is in SessionUpdate in oteapi-core.

If the strategy developer makes sure that the SessionUpdate always is JSON serialisable, the session should also be serialisable. However, it might require to introduce a new Session class.

[CasperWA]

I am unsure whether this is still relevant.

It might still be relevant depending on the data types utilized in strategy specific configurations, however, I see no issue setting a hard restriction on this for strategy (read plugin) developers to support the default JSON de/-encoder from the built-in json Python module?

In the end, pydantic offers great control of how a model is parsed to JSON through its serialization mechanism. See for example this part in the pydantic documentation.